libpng man page on SmartOS

Printed from http://www.polarhome.com/service/man/?qf=libpng&af=0&tf=2&of=SmartOS

LIBPNG(3)							     LIBPNG(3)

NAME
       libpng - Portable Network Graphics (PNG) Reference Library 1.6.10

SYNOPSIS
	#include <png.h>

       png_uint_32 png_access_version_number (void);

       void png_benign_error (png_structp png_ptr, png_const_charp error);

       void png_build_grayscale_palette (int bit_depth, png_colorp palette);

       png_voidp png_calloc (png_structp png_ptr, png_alloc_size_t size);

       void   png_chunk_benign_error   (png_structp  png_ptr,  png_const_charp
       error);

       void png_chunk_error (png_structp png_ptr, png_const_charp error);

       void png_chunk_warning (png_structp png_ptr, png_const_charp message);

       void png_convert_from_struct_tm	(png_timep  ptime,  struct  tm	FAR  *
       ttime);

       void png_convert_from_time_t (png_timep ptime, time_t ttime);

       png_charp   png_convert_to_rfc1123   (png_structp   png_ptr,  png_timep
       ptime);

       png_infop png_create_info_struct (png_structp png_ptr);

       png_structp   png_create_read_struct   (png_const_charp	 user_png_ver,
       png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

       png_structp   png_create_read_struct_2  (png_const_charp	 user_png_ver,
       png_voidp error_ptr,  png_error_ptr  error_fn,  png_error_ptr  warn_fn,
       png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

       png_structp   png_create_write_struct   (png_const_charp	 user_png_ver,
       png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

       png_structp  png_create_write_struct_2  (png_const_charp	 user_png_ver,
       png_voidp  error_ptr,  png_error_ptr  error_fn,	png_error_ptr warn_fn,
       png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

       void  png_data_freer  (png_structp  png_ptr,  png_infop	info_ptr,  int
       freer, png_uint_32 mask));

       void    png_destroy_info_struct	  (png_structp	 png_ptr,   png_infopp
       info_ptr_ptr);

       void  png_destroy_read_struct  (png_structpp  png_ptr_ptr,   png_infopp
       info_ptr_ptr, png_infopp end_info_ptr_ptr);

       void  png_destroy_write_struct  (png_structpp  png_ptr_ptr,  png_infopp
       info_ptr_ptr);

       void png_err (png_structp png_ptr);

       void png_error (png_structp png_ptr, png_const_charp error);

       void png_free (png_structp png_ptr, png_voidp ptr);

       void png_free_chunk_list (png_structp png_ptr);

       void png_free_default (png_structp png_ptr, png_voidp ptr);

       void png_free_data (png_structp png_ptr, png_infop info_ptr, int num);

       png_byte png_get_bit_depth (png_const_structp png_ptr,  png_const_infop
       info_ptr);

       png_uint_32    png_get_bKGD   (png_const_structp	  png_ptr,   png_infop
       info_ptr, png_color_16p *background);

       png_byte png_get_channels (png_const_structp  png_ptr,  png_const_infop
       info_ptr);

       png_uint_32  png_get_cHRM  (png_const_structp  png_ptr, png_const_infop
       info_ptr, double	 *white_x,  double  *white_y,  double  *red_x,	double
       *red_y,	double	*green_x,  double  *green_y,  double  *blue_x,	double
       *blue_y);

       png_uint_32     png_get_cHRM_fixed     (png_const_structp      png_ptr,
       png_const_infop	info_ptr,  png_uint_32 *white_x, png_uint_32 *white_y,
       png_uint_32   *red_x,   png_uint_32   *red_y,   png_uint_32   *green_x,
       png_uint_32 *green_y, png_uint_32 *blue_x, png_uint_32 *blue_y);

       png_uint_32   png_get_cHRM_XYZ  (png_structp  png_ptr,  png_const_infop
       info_ptr, double *red_X, double *red_Y, double *red_Z, double *green_X,
       double  *green_Y, double *green_Z, double *blue_X, double *blue_Y, dou‐
       ble *blue_Z);

       png_uint_32	png_get_cHRM_XYZ_fixed	    (png_structp      png_ptr,
       png_const_infop	info_ptr,  png_fixed_point *int_red_X, png_fixed_point
       *int_red_Y, png_fixed_point *int_red_Z,	png_fixed_point	 *int_green_X,
       png_fixed_point	   *int_green_Y,     png_fixed_point	 *int_green_Z,
       png_fixed_point	   *int_blue_X,	     png_fixed_point	  *int_blue_Y,
       png_fixed_point *int_blue_Z);

       png_uint_32 png_get_chunk_cache_max (png_const_structp png_ptr);

       png_alloc_size_t png_get_chunk_malloc_max (png_const_structp png_ptr);

       png_byte png_get_color_type (png_const_structp png_ptr, png_const_infop
       info_ptr);

       png_uint_32     png_get_compression_buffer_size	    (png_const_structp
       png_ptr);

       png_byte	    png_get_compression_type	(png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_byte png_get_copyright (png_const_structp png_ptr);

       png_uint_32 png_get_current_row_number (png_const_structp);

       png_byte png_get_current_pass_number (png_const_structp);

       png_voidp png_get_error_ptr (png_const_structp png_ptr);

       png_byte	     png_get_filter_type      (png_const_structp      png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_gAMA  (png_const_structp  png_ptr, png_const_infop
       info_ptr, double *file_gamma);

       png_uint_32     png_get_gAMA_fixed     (png_const_structp      png_ptr,
       png_const_infop info_ptr, png_uint_32 *int_file_gamma);

       png_byte png_get_header_ver (png_const_structp png_ptr);

       png_byte png_get_header_version (png_const_structp png_ptr);

       png_uint_32  png_get_hIST  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_uint_16p *hist);

       png_uint_32 png_get_iCCP	 (png_const_structp  png_ptr,  png_const_infop
       info_ptr,  png_charpp  name, int *compression_type, png_bytepp profile,
       png_uint_32 *proflen);

       png_uint_32  png_get_IHDR  (png_structp	png_ptr,  png_infop  info_ptr,
       png_uint_32   *width,   png_uint_32   *height,	int   *bit_depth,  int
       *color_type, int	 *interlace_type,  int	*compression_type,  int	 *fil‐
       ter_type);

       png_uint_32     png_get_image_height	(png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32     png_get_image_width     (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_int_32 png_get_int_32 (png_bytep buf);

       png_byte	    png_get_interlace_type     (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_uint_32 png_get_io_chunk_type (png_const_structp png_ptr);

       png_voidp png_get_io_ptr (png_structp png_ptr);

       png_uint_32 png_get_io_state (png_structp png_ptr);

       png_byte png_get_libpng_ver (png_const_structp png_ptr);

       png_voidp png_get_mem_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_oFFs	 (png_const_structp  png_ptr,  png_const_infop
       info_ptr,    png_uint_32	   *offset_x,	png_uint_32   *offset_y,   int
       *unit_type);

       png_uint_32 png_get_pCAL	 (png_const_structp  png_ptr,  png_const_infop
       info_ptr,  png_charp  *purpose,	png_int_32  *X0,  png_int_32  *X1, int
       *type, int *nparams, png_charp *units, png_charpp *params);

       png_uint_32 png_get_pHYs	 (png_const_structp  png_ptr,  png_const_infop
       info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int *unit_type);

       float	 png_get_pixel_aspect_ratio	(png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32	png_get_pHYs_dpi      (png_const_structp      png_ptr,
       png_const_infop	info_ptr,  png_uint_32 *res_x, png_uint_32 *res_y, int
       *unit_type);

       png_fixed_point	 png_get_pixel_aspect_ratio_fixed   (png_const_structp
       png_ptr, png_const_infop info_ptr);

       png_uint_32    png_get_pixels_per_inch	 (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_pixels_per_meter	(png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_voidp png_get_progressive_ptr (png_const_structp png_ptr);

       png_uint_32  png_get_PLTE  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_colorp *palette, int *num_palette);

       png_byte png_get_rgb_to_gray_status (png_const_structp png_ptr);

       png_uint_32	png_get_rowbytes      (png_const_structp      png_ptr,
       png_const_infop info_ptr);

       png_bytepp  png_get_rows	 (png_const_structp  png_ptr,  png_const_infop
       info_ptr);

       png_uint_32   png_get_sBIT   (png_const_structp	 png_ptr,    png_infop
       info_ptr, png_color_8p *sig_bit);

       void png_get_sCAL (png_const_structp png_ptr, png_const_infop info_ptr,
       int* unit, double* width, double* height);

       void  png_get_sCAL_fixed	 (png_const_structp  png_ptr,  png_const_infop
       info_ptr, int* unit, png_fixed_pointp width, png_fixed_pointp height);

       void   png_get_sCAL_s   (png_const_structp   png_ptr,   png_const_infop
       info_ptr, int* unit, png_charpp width, png_charpp height);

       png_bytep  png_get_signature  (png_const_structp	  png_ptr,   png_infop
       info_ptr);

       png_uint_32  png_get_sPLT  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_spalette_p *splt_ptr);

       png_uint_32 png_get_sRGB	 (png_const_structp  png_ptr,  png_const_infop
       info_ptr, int *file_srgb_intent);

       png_uint_32  png_get_text  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_textp *text_ptr, int *num_text);

       png_uint_32   png_get_tIME   (png_const_structp	 png_ptr,    png_infop
       info_ptr, png_timep *mod_time);

       png_uint_32    png_get_tRNS   (png_const_structp	  png_ptr,   png_infop
       info_ptr,  png_bytep  *trans_alpha,   int   *num_trans,	 png_color_16p
       *trans_color);

       /* This function is really an inline macro. */

       png_uint_16 png_get_uint_16 (png_bytep buf);

       png_uint_32 png_get_uint_31 (png_structp png_ptr, png_bytep buf);

       /* This function is really an inline macro. */

       png_uint_32 png_get_uint_32 (png_bytep buf);

       png_uint_32    png_get_unknown_chunks	(png_const_structp    png_ptr,
       png_const_infop info_ptr, png_unknown_chunkpp unknowns);

       png_voidp png_get_user_chunk_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_user_height_max (png_const_structp png_ptr);

       png_voidp png_get_user_transform_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_user_width_max (png_const_structp png_ptr);

       png_uint_32 png_get_valid (png_const_structp  png_ptr,  png_const_infop
       info_ptr, png_uint_32 flag);

       float	  png_get_x_offset_inches      (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_fixed_point	png_get_x_offset_inches_fixed  (png_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_x_offset_microns	 (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_x_offset_pixels	(png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_x_pixels_per_inch	 (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_x_pixels_per_meter	 (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       float	  png_get_y_offset_inches      (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_fixed_point	png_get_y_offset_inches_fixed  (png_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_y_offset_microns	 (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_y_offset_pixels	(png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_y_pixels_per_inch	 (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_y_pixels_per_meter	 (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);

       int   png_image_begin_read_from_file   (png_imagep  image,  const  char
       *file_name);

       int png_image_begin_read_from_stdio (png_imagep image, FILE* file);

       int,	 png_image_begin_read_from_memory      (png_imagep	image,
       png_const_voidp memory, png_size_t size);

       int  png_image_finish_read  (png_imagep	image,	png_colorp background,
       void *buffer, png_int_32 row_stride, void *colormap);

       void png_image_free (png_imagep image);

       int png_image_write_to_file (png_imagep image, const  char  *file,  int
       convert_to_8bit,	 const void *buffer, png_int_32 row_stride, void *col‐
       ormap);

       int png_image_write_to_stdio (png_imagep image, FILE  *file,  int  con‐
       vert_to_8_bit,  const  void  *buffer, png_int_32 row_stride, void *col‐
       ormap));

       void	png_info_init_3	    (png_infopp	     info_ptr,	    png_size_t
       png_info_struct_size);

       void png_init_io (png_structp png_ptr, FILE *fp);

       void png_longjmp (png_structp png_ptr, int val);

       png_voidp png_malloc (png_structp png_ptr, png_alloc_size_t size);

       png_voidp  png_malloc_default  (png_structp  png_ptr,  png_alloc_size_t
       size);

       png_voidp png_malloc_warn (png_structp png_ptr, png_alloc_size_t size);

       png_uint_32 png_permit_mng_features (png_structp	 png_ptr,  png_uint_32
       mng_features_permitted);

       void   png_process_data	 (png_structp	png_ptr,  png_infop  info_ptr,
       png_bytep buffer, png_size_t buffer_size);

       png_size_t png_process_data_pause (png_structp, int save);

       png_uint_32 png_process_data_skip (png_structp);

       void  png_progressive_combine_row   (png_structp	  png_ptr,   png_bytep
       old_row, png_bytep new_row);

       void png_read_end (png_structp png_ptr, png_infop info_ptr);

       void png_read_image (png_structp png_ptr, png_bytepp image);

       void png_read_info (png_structp png_ptr, png_infop info_ptr);

       void  png_read_png (png_structp png_ptr, png_infop info_ptr, int trans‐
       forms, png_voidp params);

       void png_read_row (png_structp png_ptr, png_bytep row,  png_bytep  dis‐
       play_row);

       void  png_read_rows  (png_structp  png_ptr,  png_bytepp row, png_bytepp
       display_row, png_uint_32 num_rows);

       void png_read_update_info (png_structp png_ptr, png_infop info_ptr);

       int png_reset_zstream (png_structp png_ptr);

       void png_save_int_32 (png_bytep buf, png_int_32 i);

       void png_save_uint_16 (png_bytep buf, unsigned int i);

       void png_save_uint_32 (png_bytep buf, png_uint_32 i);

       void png_set_add_alpha (png_structp png_ptr,  png_uint_32  filler,  int
       flags);

       void  png_set_alpha_mode	 (png_structp  png_ptr,	 int mode, double out‐
       put_gamma);

       void   png_set_alpha_mode_fixed	 (png_structp	png_ptr,   int	 mode,
       png_fixed_point output_gamma);

       void   png_set_background  (png_structp	png_ptr,  png_color_16p	 back‐
       ground_color, int background_gamma_code, int need_expand, double	 back‐
       ground_gamma);

       void png_set_background_fixed (png_structp png_ptr, png_color_16p back‐
       ground_color, int background_gamma_code, int  need_expand,  png_uint_32
       background_gamma);

       void png_set_benign_errors (png_structp png_ptr, int allowed);

       void png_set_bgr (png_structp png_ptr);

       void    png_set_bKGD    (png_structp   png_ptr,	 png_infop   info_ptr,
       png_color_16p background);

       void    png_set_check_for_invalid_index(png_structrp    png_ptr,	   int
       allowed);

       void  png_set_cHRM  (png_structp	 png_ptr,  png_infop  info_ptr, double
       white_x, double white_y, double red_x, double  red_y,  double  green_x,
       double green_y, double blue_x, double blue_y);

       void   png_set_cHRM_fixed  (png_structp	png_ptr,  png_infop  info_ptr,
       png_uint_32   white_x,	png_uint_32   white_y,	 png_uint_32	red_x,
       png_uint_32    red_y,   png_uint_32   green_x,	png_uint_32   green_y,
       png_uint_32 blue_x, png_uint_32 blue_y);

       void png_set_cHRM_XYZ (png_structp png_ptr, png_infop info_ptr,	double
       red_X, double red_Y, double red_Z, double green_X, double green_Y, dou‐
       ble green_Z, double blue_X, double blue_Y, double blue_Z);

       void png_set_cHRM_XYZ_fixed (png_structp png_ptr,  png_infop  info_ptr,
       png_fixed_point	int_red_X,  png_fixed_point int_red_Y, png_fixed_point
       int_red_Z, png_fixed_point  int_green_X,	 png_fixed_point  int_green_Y,
       png_fixed_point	    int_green_Z,      png_fixed_point	   int_blue_X,
       png_fixed_point int_blue_Y, png_fixed_point int_blue_Z);

       void   png_set_chunk_cache_max	(png_structp   png_ptr,	   png_uint_32
       user_chunk_cache_max);

       void png_set_compression_level (png_structp png_ptr, int level);

       void    png_set_compression_mem_level	(png_structp	png_ptr,   int
       mem_level);

       void png_set_compression_method (png_structp png_ptr, int method);

       void png_set_compression_strategy (png_structp png_ptr, int strategy);

       void png_set_compression_window_bits  (png_structp  png_ptr,  int  win‐
       dow_bits);

       void  png_set_crc_action	 (png_structp  png_ptr,	 int  crit_action, int
       ancil_action);

       void  png_set_error_fn  (png_structp  png_ptr,	png_voidp   error_ptr,
       png_error_ptr error_fn, png_error_ptr warning_fn);

       void png_set_expand (png_structp png_ptr);

       void png_set_expand_16 (png_structp png_ptr);

       void png_set_expand_gray_1_2_4_to_8 (png_structp png_ptr);

       void  png_set_filler  (png_structp  png_ptr,  png_uint_32  filler,  int
       flags);

       void png_set_filter (png_structp png_ptr, int method, int filters);

       void  png_set_filter_heuristics	(png_structp  png_ptr,	 int   heuris‐
       tic_method,  int	 num_weights,  png_doublep filter_weights, png_doublep
       filter_costs);

       void png_set_filter_heuristics_fixed (png_structp png_ptr, int  heuris‐
       tic_method,    int   num_weights,   png_fixed_point_p   filter_weights,
       png_fixed_point_p filter_costs);

       void png_set_flush (png_structp png_ptr, int nrows);

       void png_set_gamma (png_structp png_ptr,	 double	 screen_gamma,	double
       default_file_gamma);

       void	png_set_gamma_fixed    (png_structp    png_ptr,	   png_uint_32
       screen_gamma, png_uint_32 default_file_gamma);

       void png_set_gAMA  (png_structp	png_ptr,  png_infop  info_ptr,	double
       file_gamma);

       void   png_set_gAMA_fixed  (png_structp	png_ptr,  png_infop  info_ptr,
       png_uint_32 file_gamma);

       void png_set_gray_1_2_4_to_8 (png_structp png_ptr);

       void png_set_gray_to_rgb (png_structp png_ptr);

       void   png_set_hIST   (png_structp   png_ptr,	png_infop    info_ptr,
       png_uint_16p hist);

       void    png_set_iCCP    (png_structp   png_ptr,	 png_infop   info_ptr,
       png_const_charp name, int  compression_type,  png_const_bytep  profile,
       png_uint_32 proflen);

       int png_set_interlace_handling (png_structp png_ptr);

       void  png_set_invalid  (png_structp  png_ptr,  png_infop	 info_ptr, int
       mask);

       void png_set_invert_alpha (png_structp png_ptr);

       void png_set_invert_mono (png_structp png_ptr);

       void png_set_IHDR (png_structp png_ptr, png_infop info_ptr, png_uint_32
       width,  png_uint_32  height,  int bit_depth, int color_type, int inter‐
       lace_type, int compression_type, int filter_type);

       void  png_set_keep_unknown_chunks  (png_structp	png_ptr,   int	 keep,
       png_bytep chunk_list, int num_chunks);

       jmp_buf*	  png_set_longjmp_fn   (png_structp  png_ptr,  png_longjmp_ptr
       longjmp_fn, size_t jmp_buf_size);

       void png_set_chunk_malloc_max  (png_structp  png_ptr,  png_alloc_size_t
       user_chunk_cache_max);

       void  png_set_compression_buffer_size (png_structp png_ptr, png_uint_32
       size);

       void png_set_mem_fn (png_structp png_ptr, png_voidp  mem_ptr,  png_mal‐
       loc_ptr malloc_fn, png_free_ptr free_fn);

       void png_set_oFFs (png_structp png_ptr, png_infop info_ptr, png_uint_32
       offset_x, png_uint_32 offset_y, int unit_type);

       void png_set_packing (png_structp png_ptr);

       void png_set_packswap (png_structp png_ptr);

       void png_set_palette_to_rgb (png_structp png_ptr);

       void png_set_pCAL (png_structp png_ptr, png_infop  info_ptr,  png_charp
       purpose, png_int_32 X0, png_int_32 X1, int type, int nparams, png_charp
       units, png_charpp params);

       void png_set_pHYs (png_structp png_ptr, png_infop info_ptr, png_uint_32
       res_x, png_uint_32 res_y, int unit_type);

       void  png_set_progressive_read_fn  (png_structp png_ptr, png_voidp pro‐
       gressive_ptr, png_progressive_info_ptr info_fn, png_progressive_row_ptr
       row_fn, png_progressive_end_ptr end_fn);

       void  png_set_PLTE (png_structp png_ptr, png_infop info_ptr, png_colorp
       palette, int num_palette);

       void png_set_quantize (png_structp  png_ptr,  png_colorp	 palette,  int
       num_palette, int maximum_colors, png_uint_16p histogram, int full_quan‐
       tize);

       void png_set_read_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr
       read_data_fn);

       void  png_set_read_status_fn  (png_structp png_ptr, png_read_status_ptr
       read_row_fn);

       void   png_set_read_user_chunk_fn   (png_structp	  png_ptr,   png_voidp
       user_chunk_ptr, png_user_chunk_ptr read_user_chunk_fn);

       void	 png_set_read_user_transform_fn	     (png_structp     png_ptr,
       png_user_transform_ptr read_user_transform_fn);

       void png_set_rgb_to_gray (png_structp png_ptr, int error_action, double
       red, double green);

       void  png_set_rgb_to_gray_fixed	(png_structp png_ptr, int error_action
       png_uint_32 red, png_uint_32 green);

       void png_set_rows (png_structp png_ptr, png_infop info_ptr,  png_bytepp
       row_pointers);

       void    png_set_sBIT    (png_structp   png_ptr,	 png_infop   info_ptr,
       png_color_8p sig_bit);

       void png_set_sCAL (png_structp png_ptr, png_infop info_ptr,  int	 unit,
       double width, double height);

       void  png_set_sCAL_fixed	 (png_structp png_ptr, png_infop info_ptr, int
       unit, png_fixed_point width, png_fixed_point height);

       void png_set_sCAL_s (png_structp png_ptr, png_infop info_ptr, int unit,
       png_charp width, png_charp height);

       void png_set_scale_16 (png_structp png_ptr);

       void png_set_shift (png_structp png_ptr, png_color_8p true_bits);

       void png_set_sig_bytes (png_structp png_ptr, int num_bytes);

       void    png_set_sPLT    (png_structp   png_ptr,	 png_infop   info_ptr,
       png_spalette_p splt_ptr, int num_spalettes);

       void  png_set_sRGB  (png_structp	 png_ptr,  png_infop   info_ptr,   int
       srgb_intent);

       void   png_set_sRGB_gAMA_and_cHRM   (png_structp	  png_ptr,   png_infop
       info_ptr, int srgb_intent);

       void png_set_strip_16 (png_structp png_ptr);

       void png_set_strip_alpha (png_structp png_ptr);

       void  png_set_strip_error_numbers  (png_structp	png_ptr,   png_uint_32
       strip_mode);

       void png_set_swap (png_structp png_ptr);

       void png_set_swap_alpha (png_structp png_ptr);

       void  png_set_text  (png_structp png_ptr, png_infop info_ptr, png_textp
       text_ptr, int num_text);

       void png_set_text_compression_level (png_structp png_ptr, int level);

       void  png_set_text_compression_mem_level	 (png_structp	png_ptr,   int
       mem_level);

       void png_set_text_compression_strategy (png_structp png_ptr, int strat‐
       egy);

       void  png_set_text_compression_window_bits  (png_structp	 png_ptr,  int
       window_bits);

       void   png_set_text_compression_method,	 (png_structp	png_ptr,   int
       method));

       void png_set_tIME (png_structp png_ptr, png_infop  info_ptr,  png_timep
       mod_time);

       void  png_set_tRNS  (png_structp png_ptr, png_infop info_ptr, png_bytep
       trans_alpha, int num_trans, png_color_16p trans_color);

       void png_set_tRNS_to_alpha (png_structp png_ptr);

       png_uint_32  png_set_unknown_chunks  (png_structp  png_ptr,   png_infop
       info_ptr, png_unknown_chunkp unknowns, int num, int location);

       void  png_set_unknown_chunk_location  (png_structp  png_ptr,  png_infop
       info_ptr, int chunk, int location);

       void    png_set_user_limits    (png_structp    png_ptr,	   png_uint_32
       user_width_max, png_uint_32 user_height_max);

       void   png_set_user_transform_info   (png_structp   png_ptr,  png_voidp
       user_transform_ptr, int user_transform_depth, int  user_transform_chan‐
       nels);

       void   png_set_write_fn	 (png_structp	png_ptr,   png_voidp   io_ptr,
       png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);

       void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr
       write_row_fn);

       void	 png_set_write_user_transform_fn     (png_structp     png_ptr,
       png_user_transform_ptr write_user_transform_fn);

       int  png_sig_cmp	  (png_bytep   sig,   png_size_t   start,   png_size_t
       num_to_check);

       void png_start_read_image (png_structp png_ptr);

       void png_warning (png_structp png_ptr, png_const_charp message);

       void   png_write_chunk	(png_structp  png_ptr,	png_bytep  chunk_name,
       png_bytep data, png_size_t length);

       void  png_write_chunk_data  (png_structp	  png_ptr,   png_bytep	 data,
       png_size_t length);

       void png_write_chunk_end (png_structp png_ptr);

       void  png_write_chunk_start (png_structp png_ptr, png_bytep chunk_name,
       png_uint_32 length);

       void png_write_end (png_structp png_ptr, png_infop info_ptr);

       void png_write_flush (png_structp png_ptr);

       void png_write_image (png_structp png_ptr, png_bytepp image);

       void png_write_info (png_structp png_ptr, png_infop info_ptr);

       void   png_write_info_before_PLTE   (png_structp	  png_ptr,   png_infop
       info_ptr);

       void png_write_png (png_structp png_ptr, png_infop info_ptr, int trans‐
       forms, png_voidp params);

       void png_write_row (png_structp png_ptr, png_bytep row);

       void png_write_rows (png_structp png_ptr, png_bytepp  row,  png_uint_32
       num_rows);

       void png_write_sig (png_structp png_ptr);

DESCRIPTION
       The  libpng  library supports encoding, decoding, and various manipula‐
       tions of the Portable Network Graphics (PNG) format  image  files.   It
       uses  the  zlib(3)  compression	library.   Following  is a copy of the
       libpng-manual.txt file that accompanies libpng.

LIBPNG.TXT
       libpng-manual.txt - A description on how to use and modify libpng

	libpng version 1.6.10 - March 6, 2014
	Updated and distributed by Glenn Randers-Pehrson
	<glennrp at users.sourceforge.net>
	Copyright (c) 1998-2014 Glenn Randers-Pehrson

	This document is released under the libpng license.
	For conditions of distribution and use, see the disclaimer
	and license in png.h

	Based on:

	libpng versions 0.97, January 1998, through 1.6.10 - March 6, 2014
	Updated and distributed by Glenn Randers-Pehrson
	Copyright (c) 1998-2014 Glenn Randers-Pehrson

	libpng 1.0 beta 6  version 0.96 May 28, 1997
	Updated and distributed by Andreas Dilger
	Copyright (c) 1996, 1997 Andreas Dilger

	libpng 1.0 beta 2 - version 0.88  January 26, 1996
	For conditions of distribution and use, see copyright
	notice in png.h. Copyright (c) 1995, 1996 Guy Eric
	Schalnat, Group 42, Inc.

	Updated/rewritten per request in the libpng FAQ
	Copyright (c) 1995, 1996 Frank J. T. Wojcik
	December 18, 1995 & January 20, 1996

	TABLE OF CONTENTS

	   I. Introduction
	  II. Structures
	 III. Reading
	  IV. Writing
	   V. Simplified API
	  VI. Modifying/Customizing libpng
	 VII. MNG support
	VIII. Changes to Libpng from version 0.88
	  IX. Changes to Libpng from version 1.0.x to 1.2.x
	   X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
	  XI. Changes to Libpng from version 1.4.x to 1.5.x
	 XII. Changes to Libpng from version 1.5.x to 1.6.x
	XIII. Detecting libpng
	 XIV. Source code repository
	  XV. Coding style
	 XVI. Y2K Compliance in libpng

I. Introduction
       This file describes how to use and modify  the  PNG  reference  library
       (known  as  libpng)  for your own use.  In addition to this file, exam‐
       ple.c is a good starting point for using the library, as it is  heavily
       commented  and  should  include	everything  most people will need.  We
       assume that libpng is already  installed;  see  the  INSTALL  file  for
       instructions on how to install libpng.

       For  examples  of libpng usage, see the files "example.c", "pngtest.c",
       and the files in the "contrib" directory, all of which are included  in
       the libpng distribution.

       Libpng was written as a companion to the PNG specification, as a way of
       reducing the amount of time and effort it takes to support the PNG file
       format in application programs.

       The  PNG specification (second edition), November 2003, is available as
       a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at
       <http://www.w3.org/TR/2003/REC-PNG-20031110/  The W3C and ISO documents
       have identical technical content.

       The	 PNG-1.2       specification	   is	    available	    at
       <http://www.libpng.org/pub/png/documents/>.   It is technically equiva‐
       lent to the PNG specification (second edition) but has some  additional
       material.

       The    PNG-1.0	 specification	  is	available    as	   RFC	  2083
       <http://www.libpng.org/pub/png/documents/> and as a W3C	Recommendation
       <http://www.w3.org/TR/REC.png.html>.

       Some  additional	 chunks	 are  described	 in the special-purpose public
       chunks documents at <http://www.libpng.org/pub/png/documents/>.

       Other information about PNG, and the latest version of libpng,  can  be
       found at the PNG home page, <http://www.libpng.org/pub/png/>.

       Most  users will not have to modify the library significantly; advanced
       users may want to modify it more.  All attempts were made to make it as
       complete	 as possible, while keeping the code easy to understand.  Cur‐
       rently, this library only supports C.  Support for other	 languages  is
       being considered.

       Libpng has been designed to handle multiple sessions at one time, to be
       easily modifiable, to be portable to  the  vast	majority  of  machines
       (ANSI,  K&R,  16-,  32-,	 and 64-bit) available, and to be easy to use.
       The ultimate goal of libpng is to promote the  acceptance  of  the  PNG
       file  format in whatever way possible.  While there is still work to be
       done (see the TODO file), libpng should cover the majority of the needs
       of its users.

       Libpng  uses  zlib  for its compression and decompression of PNG files.
       Further information about zlib, and the latest version of zlib, can  be
       found	  at	 the	 zlib	  home	   page,     <http://www.info-
       zip.org/pub/infozip/zlib/>.  The zlib compression utility is a  general
       purpose utility that is useful for more than PNG files, and can be used
       without libpng.	See the documentation delivered	 with  zlib  for  more
       details.	  You  can  usually find the source files for the zlib utility
       wherever you find the libpng source files.

       Libpng is  thread  safe,	 provided  the	threads	 are  using  different
       instances   of  the  structures.	  Each	thread	should	have  its  own
       png_struct and png_info instances, and thus its own image.  Libpng does
       not  protect  itself  against  two threads using the same instance of a
       structure.

II. Structures
       There are two main structures that are important to libpng,  png_struct
       and  png_info.  Both are internal structures that are no longer exposed
       in the libpng interface (as of libpng 1.5.0).

       The png_info structure is designed to provide information about the PNG
       file.  At one time, the fields of png_info were intended to be directly
       accessible to the user.	However, this tended to	 cause	problems  with
       applications  using dynamically loaded libraries, and as a result a set
       of interface functions for png_info (the	 png_get_*()  and  png_set_*()
       functions)  was developed, and direct access to the png_info fields was
       deprecated..

       The png_struct structure is the object used by the library to decode  a
       single image.  As of 1.5.0 this structure is also not exposed.

       Almost  all  libpng APIs require a pointer to a png_struct as the first
       argument.  Many (in particular  the  png_set  and  png_get  APIs)  also
       require a pointer to png_info as the second argument.  Some application
       visible macros defined in png.h designed for basic data access (reading
       and  writing integers in the PNG format) don't take a png_info pointer,
       but it's almost always safe to assume that a (png_struct*)  has	to  be
       passed to call an API function.

       You can have more than one png_info structure associated with an image,
       as illustrated in pngtest.c, one for information	 valid	prior  to  the
       IDAT  chunks  and  another  (called  "end_info" below) for things after
       them.

       The png.h header file is an invaluable reference for  programming  with
       libpng.	 And  while I'm on the topic, make sure you include the libpng
       header file:

       #include <png.h>

       and also (as of libpng-1.5.0) the zlib header file, if you need it:

       #include <zlib.h>

   Types
       The png.h header file defines a number of integral types	 used  by  the
       APIs.   Most of these are fairly obvious; for example types correspond‐
       ing to integers of particular sizes and types for passing color values.

       One exception is how non-integral numbers are handled.  For application
       convenience most APIs that take such numbers have C (double) arguments;
       however, internally PNG, and libpng, use 32  bit	 signed	 integers  and
       encode  the value by multiplying by 100,000.  As of libpng 1.5.0 a con‐
       venience	 macro	PNG_FP_1  is  defined  in  png.h  along	 with  a  type
       (png_fixed_point) which is simply (png_int_32).

       All  APIs  that	take  (double) arguments also have a matching API that
       takes the corresponding fixed point integer arguments.  The fixed point
       API has the same name as the floating point one with "_fixed" appended.
       The actual range of values permitted in the  APIs  is  frequently  less
       than the full range of (png_fixed_point) (-21474 to +21474).  When APIs
       require a non-negative argument the type	 is  recorded  as  png_uint_32
       above.	Consult	 the  header file and the text below for more informa‐
       tion.

       Special care must be take with sCAL chunk handling  because  the	 chunk
       itself  uses  non-integral values encoded as strings containing decimal
       floating point numbers.	See the comments in the header file.

   Configuration
       The main header file function declarations are frequently protected  by
       C preprocessing directives of the form:

	   #ifdef PNG_feature_SUPPORTED
	   declare-function
	   #endif
	   ...
	   #ifdef PNG_feature_SUPPORTED
	   use-function
	   #endif

       The  library  can  be  built without support for these APIs, although a
       standard build will have all implemented	 APIs.	 Application  programs
       should  check the feature macros before using an API for maximum porta‐
       bility.	From libpng 1.5.0 the feature macros set during the  build  of
       libpng  are recorded in the header file "pnglibconf.h" and this file is
       always included by png.h.

       If you don't need to change the library configuration from the default,
       skip to the next section ("Reading").

       Notice  that  some  of the makefiles in the 'scripts' directory and (in
       1.5.0) all of the build project files in the 'projects' directory  sim‐
       ply  copy  scripts/pnglibconf.h.prebuilt	 to  pnglibconf.h.  This means
       that these build systems do not permit easy auto-configuration  of  the
       library - they only support the default configuration.

       The  easiest way to make minor changes to the libpng configuration when
       auto-configuration is supported is to add definitions  to  the  command
       line using (typically) CPPFLAGS.	 For example:

       CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC

       will  change  the internal libpng math implementation for gamma correc‐
       tion and other arithmetic calculations to  fixed	 point,	 avoiding  the
       need  for  fast	floating point support.	 The result can be seen in the
       generated pnglibconf.h - make sure  it  contains	 the  changed  feature
       macro setting.

       If  you	need  to make more extensive configuration changes - more than
       one or two feature macro settings - you can either add  -DPNG_USER_CON‐
       FIG  to the build command line and put a list of feature macro settings
       in pngusr.h or you can set DFA_XTRA (a makefile	variable)  to  a  file
       containing the same information in the form of 'option' settings.

       A. Changing pnglibconf.h

       A  variety  of methods exist to build libpng.  Not all of these support
       reconfiguration of pnglibconf.h.	 To reconfigure pnglibconf.h  it  must
       either  be  rebuilt from scripts/pnglibconf.dfa using awk or it must be
       edited by hand.

       Hand editing is achieved by  copying  scripts/pnglibconf.h.prebuilt  to
       pnglibconf.h  and  changing  the lines defining the supported features,
       paying  very  close  attention	to   the   'option'   information   in
       scripts/pnglibconf.dfa that describes those features and their require‐
       ments.  This is easy to get wrong.

       B. Configuration using DFA_XTRA

       Rebuilding from pnglibconf.dfa is easy if a  functioning	 'awk',	 or  a
       later  variant  such  as 'nawk' or 'gawk', is available.	 The configure
       build will automatically find an	 appropriate  awk  and	build  pnglib‐
       conf.h.	 The  scripts/pnglibconf.mak file contains a set of make rules
       for doing the same thing if configure is not  used,  and	 many  of  the
       makefiles in the scripts directory use this approach.

       When  rebuilding simply write a new file containing changed options and
       set DFA_XTRA to the name of this file.  This causes the build to append
       the new file to the end of scripts/pnglibconf.dfa.  The pngusr.dfa file
       should contain lines of the following forms:

       everything = off

       This turns all optional features off.   Include	it  at	the  start  of
       pngusr.dfa  to  make  it	 easier to build a minimal configuration.  You
       will need to turn at least some features on afterward to enable	either
       reading or writing code, or both.

       option feature on option feature off

       Enable  or  disable  a  single feature.	This will automatically enable
       other features required by a feature that is turned on or disable other
       features	 that require a feature which is turned off.  Conflicting set‐
       tings will cause an error message to be emitted by awk.

       setting feature default value

       Changes the default value of setting 'feature' to 'value'.  There are a
       small  number  of  settings listed at the top of pnglibconf.h, they are
       documented in the source code.  Most of these values  have  performance
       implications for the library but most of them have no visible effect on
       the API.	 Some can also be overridden from the API.

       This method of building a customized  pnglibconf.h  is  illustrated  in
       contrib/pngminim/*.   See  the "$(PNGCONF):" target in the makefile and
       pngusr.dfa in these directories.

       C. Configuration using PNG_USER_CONFIG

       If -DPNG_USER_CONFIG is added to	 the  CPPFLAGS	when  pnglibconf.h  is
       built,  the  file  pngusr.h  will  automatically be included before the
       options in scripts/pnglibconf.dfa are processed.	  Your	pngusr.h  file
       should  contain	only  macro  definitions turning features on or off or
       setting settings.

       Apart from the global setting "everything = off" all the options listed
       above can be set using macros in pngusr.h:

       #define PNG_feature_SUPPORTED

       is equivalent to:

       option feature on

       #define PNG_NO_feature

       is equivalent to:

       option feature off

       #define PNG_feature value

       is equivalent to:

       setting feature default value

       Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
       pngusr file you supply override the contents of scripts/pnglibconf.dfa

       If confusing or incomprehensible behavior results  it  is  possible  to
       examine	the  intermediate  file pnglibconf.dfn to find the full set of
       dependency information for each setting and option.  Simply locate  the
       feature in the file and read the C comments that precede it.

       This method is also illustrated in the contrib/pngminim/* makefiles and
       pngusr.h.

III. Reading
       We'll now walk you through the possible functions to call when  reading
       in  a  PNG file sequentially, briefly explaining the syntax and purpose
       of each one.  See example.c and png.h for more detail.  While  progres‐
       sive  reading  is covered in the next section, you will still need some
       of the functions discussed in this section to read a PNG file.

   Setup
       You will want to do the	I/O  initialization(*)	before	you  get  into
       libpng, so if it doesn't work, you don't have much to undo.  Of course,
       you will also want to insure that you are, in fact, dealing with a  PNG
       file.   Libpng  provides a simple check to see if a file is a PNG file.
       To use it, pass in the first 1 to 8 bytes of the file to	 the  function
       png_sig_cmp(), and it will return 0 (false) if the bytes match the cor‐
       responding bytes of the PNG signature, or nonzero (true) otherwise.  Of
       course,	the  more  bytes  you pass in, the greater the accuracy of the
       prediction.

       If you are intending to keep the file pointer open for use  in  libpng,
       you  must ensure you don't read more than 8 bytes from the beginning of
       the file, and you also have to make a call to  png_set_sig_bytes_read()
       with the number of bytes you read from the beginning.  Libpng will then
       only check the bytes (if any) that your program didn't read.

       (*): If you are not using the standard I/O functions, you will need  to
       replace them with custom functions.  See the discussion under Customiz‐
       ing libpng.

	   FILE *fp = fopen(file_name, "rb");
	   if (!fp)
	   {
	      return (ERROR);
	   }

	   fread(header, 1, number, fp);
	   is_png = !png_sig_cmp(header, 0, number);

	   if (!is_png)
	   {
	      return (NOT_PNG);
	   }

       Next, png_struct and png_info need to be allocated and initialized.  In
       order  to ensure that the size of these structures is correct even with
       a dynamically linked libpng, there  are	functions  to  initialize  and
       allocate	 the  structures.   We also pass the library version, optional
       pointers to error handling functions, and a pointer to  a  data	struct
       for use by the error functions, if necessary (the pointer and functions
       can be NULL if the default error handlers are to	 be  used).   See  the
       section	on  Changes  to	 Libpng below regarding the old initialization
       functions.  The structure allocation functions quietly return  NULL  if
       they fail to create the structure, so your application should check for
       that.

	   png_structp png_ptr = png_create_read_struct
	       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
	       user_error_fn, user_warning_fn);

	   if (!png_ptr)
	      return (ERROR);

	   png_infop info_ptr = png_create_info_struct(png_ptr);

	   if (!info_ptr)
	   {
	      png_destroy_read_struct(&png_ptr,
		  (png_infopp)NULL, (png_infopp)NULL);
	      return (ERROR);
	   }

       If you want to use your own memory allocation routines,	use  a	libpng
       that  was  built	 with PNG_USER_MEM_SUPPORTED defined, and use png_cre‐
       ate_read_struct_2() instead of png_create_read_struct():

	   png_structp png_ptr = png_create_read_struct_2
	       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
	       user_error_fn, user_warning_fn, (png_voidp)
	       user_mem_ptr, user_malloc_fn, user_free_fn);

       The error handling routines passed to png_create_read_struct() and  the
       memory  alloc/free  routines  passed  to png_create_struct_2() are only
       necessary if you are not using the libpng supplied error	 handling  and
       memory alloc/free functions.

       When  libpng  encounters	 an  error, it expects to longjmp back to your
       routine.	 Therefore, you	 will  need  to	 call  setjmp  and  pass  your
       png_jmpbuf(png_ptr).  If you read the file from different routines, you
       will need to update the longjmp buffer every time you enter a new  rou‐
       tine that will call a png_*() function.

       See  your  documentation	 of  setjmp/longjmp for your compiler for more
       information on setjmp/longjmp.  See the discussion on libpng error han‐
       dling  in  the Customizing Libpng section below for more information on
       the libpng error handling.  If an error occurs,	and  libpng  longjmp's
       back to your setjmp, you will want to call png_destroy_read_struct() to
       free any memory.

	   if (setjmp(png_jmpbuf(png_ptr)))
	   {
	      png_destroy_read_struct(&png_ptr, &info_ptr,
		  &end_info);
	      fclose(fp);
	      return (ERROR);
	   }

       Pass (png_infopp)NULL instead of &end_info  if  you  didn't  create  an
       end_info structure.

       If  you would rather avoid the complexity of setjmp/longjmp issues, you
       can compile libpng with PNG_NO_SETJMP, in which case errors will result
       in a call to PNG_ABORT() which defaults to abort().

       You can #define PNG_ABORT() to a function that does something more use‐
       ful than abort(), as long as your function does not return.

       Now you need to set up the input code.  The default for	libpng	is  to
       use  the	 C function fread().  If you use this, you will need to pass a
       valid FILE * in the function png_init_io().  Be sure that the  file  is
       opened  in  binary mode.	 If you wish to handle reading data in another
       way, you need not call the png_init_io() function, but  you  must  then
       implement  the  libpng  I/O methods discussed in the Customizing Libpng
       section below.

	   png_init_io(png_ptr, fp);

       If you had previously opened the file and read  any  of	the  signature
       from  the beginning in order to see if this was a PNG file, you need to
       let libpng know that there are some bytes missing from the start of the
       file.

	   png_set_sig_bytes(png_ptr, number);

       You  can change the zlib compression buffer size to be used while read‐
       ing compressed data with

	   png_set_compression_buffer_size(png_ptr, buffer_size);

       where the default size is 8192 bytes.  Note that	 the  buffer  size  is
       changed	immediately and the buffer is reallocated immediately, instead
       of setting a flag to be acted upon later.

       If you want CRC errors to be handled in a  different  manner  than  the
       default, use

	   png_set_crc_action(png_ptr, crit_action, ancil_action);

       The  values  for	 png_set_crc_action()  say how libpng is to handle CRC
       errors in ancillary and critical chunks, and whether to	use  the  data
       contained  therein.   Note that it is impossible to "discard" data in a
       critical chunk.

       Choices for (int) crit_action are
	  PNG_CRC_DEFAULT      0  error/quit
	  PNG_CRC_ERROR_QUIT   1  error/quit
	  PNG_CRC_WARN_USE     3  warn/use data
	  PNG_CRC_QUIET_USE    4  quiet/use data
	  PNG_CRC_NO_CHANGE    5  use the current value

       Choices for (int) ancil_action are
	  PNG_CRC_DEFAULT      0  error/quit
	  PNG_CRC_ERROR_QUIT   1  error/quit
	  PNG_CRC_WARN_DISCARD 2  warn/discard data
	  PNG_CRC_WARN_USE     3  warn/use data
	  PNG_CRC_QUIET_USE    4  quiet/use data
	  PNG_CRC_NO_CHANGE    5  use the current value

   Setting up callback code
       You can set up a callback function to handle any unknown chunks in  the
       input stream. You must supply the function

	   read_chunk_callback(png_structp png_ptr,
		png_unknown_chunkp chunk);
	   {
	      /* The unknown chunk structure contains your
		 chunk data, along with similar data for any other
		 unknown chunks: */

		  png_byte name[5];
		  png_byte *data;
		  png_size_t size;

	      /* Note that libpng has already taken care of
		 the CRC handling */

	      /* put your code here.  Search for your chunk in the
		 unknown chunk structure, process it, and return one
		 of the following: */

	      return (-n); /* chunk had an error */
	      return (0); /* did not recognize */
	      return (n); /* success */
	   }

       (You  can  give	your  function	another	 name that you like instead of
       "read_chunk_callback")

       To inform libpng about your function, use

	   png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
	       read_chunk_callback);

       This names not only the callback function, but also a user pointer that
       you can retrieve with

	   png_get_user_chunk_ptr(png_ptr);

       If you call the png_set_read_user_chunk_fn() function, then all unknown
       chunks which the callback does not handle will be saved when read.  You
       can  cause them to be discarded by returning '1' ("handled") instead of
       '0'.  This behavior will change in libpng 1.7 and the default  handling
       set  by	the  png_set_keep_unknown_chunks()  function, described below,
       will be used when the callback returns 0.  If  you  want	 the  existing
       behavior	 you should set the global default to PNG_HANDLE_CHUNK_IF_SAFE
       now; this is compatible with all current versions of  libpng  and  with
       1.7.   Libpng 1.6 issues a warning if you keep the default, or PNG_HAN‐
       DLE_CHUNK_NEVER, and the callback returns 0.

       At this point, you can set up a callback function that will  be	called
       after  each  row has been read, which you can use to control a progress
       meter or the like.  It's demonstrated in pngtest.c.  You must supply  a
       function

	   void read_row_callback(png_structp png_ptr,
	      png_uint_32 row, int pass);
	   {
	     /* put your code here */
	   }

       (You  can give it another name that you like instead of "read_row_call‐
       back")

       To inform libpng about your function, use

	   png_set_read_status_fn(png_ptr, read_row_callback);

       When this function is called the row has already been  completely  pro‐
       cessed  and  the	 'row' and 'pass' refer to the next row to be handled.
       For the non-interlaced case the row that was just handled is simply one
       less than the passed in row number, and pass will always be 0.  For the
       interlaced case the same applies unless the row value is	 0,  in	 which
       case  the  row  just handled was the last one from one of the preceding
       passes.	Because interlacing may skip a pass you cannot	be  sure  that
       the  preceding  pass  is just 'pass-1', if you really need to know what
       the last pass is record (row,pass) from the callback and use  the  last
       recorded value each time.

       As  with	 the  user  transform  you  can	 find the output row using the
       PNG_ROW_FROM_PASS_ROW macro.

   Unknown-chunk handling
       Now you get to set the way the library processes unknown chunks in  the
       input  PNG  stream. Both known and unknown chunks will be read.	Normal
       behavior is that known chunks will be parsed into information in	 vari‐
       ous  info_ptr  members  while  unknown  chunks  will be discarded. This
       behavior can be wasteful if your application will never use some	 known
       chunk types. To change this, you can call:

	   png_set_keep_unknown_chunks(png_ptr, keep,
	       chunk_list, num_chunks);

	   keep	      - 0: default unknown chunk handling
			1: ignore; do not keep
			2: keep only if safe-to-copy
			3: keep even if unsafe-to-copy

		      You can use these definitions:
			PNG_HANDLE_CHUNK_AS_DEFAULT   0
			PNG_HANDLE_CHUNK_NEVER	      1
			PNG_HANDLE_CHUNK_IF_SAFE      2
			PNG_HANDLE_CHUNK_ALWAYS	      3

	   chunk_list - list of chunks affected (a byte string,
			five bytes per chunk, NULL or ' ' if
			num_chunks is positive; ignored if
			numchunks <= 0).

	   num_chunks - number of chunks affected; if 0, all
			unknown chunks are affected.  If positive,
			only the chunks in the list are affected,
			and if negative all unknown chunks and
			all known chunks except for the IHDR,
			PLTE, tRNS, IDAT, and IEND chunks are
			affected.

       Unknown	chunks	declared  in this way will be saved as raw data onto a
       list of png_unknown_chunk structures.  If  a  chunk  that  is  normally
       known  to  libpng  is named in the list, it will be handled as unknown,
       according to the "keep" directive.  If a chunk is named	in  successive
       instances  of  png_set_keep_unknown_chunks(),  the  final instance will
       take precedence.	 The IHDR and IEND  chunks  should  not	 be  named  in
       chunk_list;  if they are, libpng will process them normally anyway.  If
       you know that your application will never make use of  some  particular
       chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.

       Here is an example of the usage of png_set_keep_unknown_chunks(), where
       the private "vpAg" chunk will later be processed by a user chunk	 call‐
       back function:

	   png_byte vpAg[5]={118, 112,	65, 103, (png_byte) ' '};

	   #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
	     png_byte unused_chunks[]=
	     {
	       104,  73,  83,  84, (png_byte) ' ',   /* hIST */
	       105,  84,  88, 116, (png_byte) ' ',   /* iTXt */
	       112,  67,  65,  76, (png_byte) ' ',   /* pCAL */
	       115,  67,  65,  76, (png_byte) ' ',   /* sCAL */
	       115,  80,  76,  84, (png_byte) ' ',   /* sPLT */
	       116,  73,  77,  69, (png_byte) ' ',   /* tIME */
	     };
	   #endif

	   ...

	   #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
	     /* ignore all unknown chunks
	      * (use global setting "2" for libpng16 and earlier):
	      */
	     png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0);

	     /* except for vpAg: */
	     png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);

	     /* also ignore unused known chunks: */
	     png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
		(int)(sizeof unused_chunks)/5);
	   #endif

   User limits
       The  PNG specification allows the width and height of an image to be as
       large as 2^(31-1 (0x7fffffff), or about 2.147 billion rows and columns.
       Since  very  few applications really need to process such large images,
       we have imposed an arbitrary  1-million	limit  on  rows	 and  columns.
       Larger  images will be rejected immediately with a png_error() call. If
       you wish to change this limit, you can use

	  png_set_user_limits(png_ptr, width_max, height_max);

       to set your own limits, or use width_max = height_max = 0x7fffffffL  to
       allow  all  valid  dimensions (libpng may reject some very large images
       anyway because of potential buffer overflow conditions).

       You should put this statement after you create the  PNG	structure  and
       before calling png_read_info(), png_read_png(), or png_process_data().

       When  writing  a	 PNG  datastream,  put	this  statement before calling
       png_write_info() or png_write_png().

       If you need to retrieve the limits that are being applied, use

	  width_max = png_get_user_width_max(png_ptr);
	  height_max = png_get_user_height_max(png_ptr);

       The PNG specification sets no limit on the number of  ancillary	chunks
       allowed	in a PNG datastream.  You can impose a limit on the total num‐
       ber of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be  stored,
       with

	  png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

       where 0x7fffffffL means unlimited.  You can retrieve this limit with

	  chunk_cache_max = png_get_chunk_cache_max(png_ptr);

       You  can	 also  set  a  limit on the amount of memory that a compressed
       chunk other than IDAT can occupy, with

	  png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);

       and you can retrieve the limit with

	  chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);

       Any chunks that would cause either of these limits to be exceeded  will
       be ignored.

   Information about your system
       If  you	intend	to display the PNG or to incorporate it in other image
       data you need to tell libpng information about your display or  drawing
       surface so that libpng can convert the values in the image to match the
       display.

       From libpng-1.5.4 this information can be set before  reading  the  PNG
       file  header.   In earlier versions png_set_gamma() existed but behaved
       incorrectly if called before the PNG file  header  had  been  read  and
       png_set_alpha_mode() did not exist.

       If  you need to support versions prior to libpng-1.5.4 test the version
       number as illustrated below using "PNG_LIBPNG_VER >= 10504" and	follow
       the procedures described in the appropriate manual page.

       You  give  libpng  the  encoding expected by your system expressed as a
       'gamma' value.  You can also specify a default  encoding	 for  the  PNG
       file  in	 case  the  required information is missing from the file.  By
       default libpng assumes that the PNG data matches your system,  to  keep
       this default call:

	  png_set_gamma(png_ptr, screen_gamma, output_gamma);

       or you can use the fixed point equivalent:

	  png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
	     PNG_FP_1*output_gamma);

       If you don't know the gamma for your system it is probably 2.2 - a good
       approximation to the IEC	 standard  for	display	 systems  (sRGB).   If
       images  are too contrasty or washed out you got the value wrong - check
       your system documentation!

       Many systems permit the system gamma to be changed via a	 lookup	 table
       in  the display driver, a few systems, including older Macs, change the
       response by default.  As of 1.5.4 three special values are available to
       handle common situations:

	  PNG_DEFAULT_sRGB: Indicates that the system conforms to the
			    IEC 61966-2-1 standard.  This matches almost
			    all systems.
	  PNG_GAMMA_MAC_18: Indicates that the system is an older
			    (pre Mac OS 10.6) Apple Macintosh system with
			    the default settings.
	  PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates
			    that the system expects data with no gamma
			    encoding.

       You  would  use the linear (unencoded) value if you need to process the
       pixel values further because this avoids the need  to  decode  and  re-
       encode each component value whenever arithmetic is performed.  A lot of
       graphics software uses linear values for this reason, often with higher
       precision component values to preserve overall accuracy.

       The  output_gamma  value expresses how to decode the output values, not
       how they are encoded.  The values used correspond to the normal numbers
       used  to	 describe  the overall gamma of a computer display system; for
       example 2.2 for an sRGB conformant system.  The values  are  scaled  by
       100000 in the _fixed version of the API (so 220000 for sRGB.)

       The  inverse  of	 the value is always used to provide a default for the
       PNG file encoding if it has no gAMA chunk and  if  png_set_gamma()  has
       not been called to override the PNG gamma information.

       When  the  ALPHA_OPTIMIZED mode is selected the output gamma is used to
       encode opaque pixels however pixels with lower  alpha  values  are  not
       encoded, regardless of the output gamma setting.

       When  the  standard  Porter  Duff handling is requested with mode 1 the
       output encoding is set to be linear and the output_gamma value is  only
       relevant	 as  a	default	 for input data that has no gamma information.
       The linear output encoding will be  overridden  if  png_set_gamma()  is
       called - the results may be highly unexpected!

       The  following  numbers	are  derived  from  the	 sRGB standard and the
       research behind it.  sRGB is defined to be approximated by a  PNG  gAMA
       chunk  value of 0.45455 (1/2.2) for PNG.	 The value implicitly includes
       any viewing correction required to take account of any  differences  in
       the  color  environment	of the original scene and the intended display
       environment; the value expresses how to *decode* the image for display,
       not how the original data was *encoded*.

       sRGB provides a peg for the PNG standard by defining a viewing environ‐
       ment.  sRGB itself, and earlier TV standards, actually use a more  com‐
       plex  transform	(a linear portion then a gamma 2.4 power law) than PNG
       can express.  (PNG is limited to simple power laws.)  By saying that an
       image  for direct display on an sRGB conformant system should be stored
       with a gAMA chunk value of 45455 (11.3.3.2 and 11.3.3.5 of the ISO  PNG
       specification) the PNG specification makes it possible to derive values
       for other display systems and environments.

       The Mac value is deduced from the sRGB based on an assumption that  the
       actual  extra  viewing correction used in early Mac display systems was
       implemented as a power 1.45 lookup table.

       Any system where a programmable lookup  table  is  used	or  where  the
       behavior	 of  the  final	 display device characteristics can be changed
       requires system specific code to	 obtain	 the  current  characteristic.
       However	this  can  be  difficult  and  most  PNG gamma correction only
       requires an approximate value.

       By default, if png_set_alpha_mode() is not called, libpng assumes  that
       all  values  are	 unencoded,  linear, values and that the output device
       also has a linear characteristic.  This is only very rarely  correct  -
       it   is	 invariably   better   to   call   png_set_alpha_mode()	  with
       PNG_DEFAULT_sRGB than rely on the default if you don't  know  what  the
       right answer is!

       The  special  value PNG_GAMMA_MAC_18 indicates an older Mac system (pre
       Mac OS 10.6) which used a correction  table  to	implement  a  somewhat
       lower gamma on an otherwise sRGB system.

       Both  these  values  are reserved (not simple gamma values) in order to
       allow more precise correction internally in the future.

       NOTE: the values can be passed to either the fixed  or  floating	 point
       APIs,  but  the floating point API will also accept floating point val‐
       ues.

       The second thing you may need to tell libpng about is how  your	system
       handles	alpha  channel information.  Some, but not all, PNG files con‐
       tain an alpha channel.  To display these files correctly	 you  need  to
       compose	the  data  onto a suitable background, as described in the PNG
       specification.

       Libpng only supports composing onto a single color (using png_set_back‐
       ground;	see  below).   Otherwise  you must do the composition yourself
       and, in this case, you may need to call png_set_alpha_mode:

	  #if PNG_LIBPNG_VER >= 10504
	     png_set_alpha_mode(png_ptr, mode, screen_gamma);
	  #else
	     png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
	  #endif

       The screen_gamma value is the same as the  argument  to	png_set_gamma;
       however,	  how	it   affects   the   output   depends	on  the	 mode.
       png_set_alpha_mode() sets the file gamma default to 1/screen_gamma,  so
       normally	 you  don't need to call png_set_gamma.	 If you need different
       defaults call png_set_gamma() before png_set_alpha_mode() - if you call
       it after it will override the settings made by png_set_alpha_mode().

       The mode is as follows:

	   PNG_ALPHA_PNG:  The data is encoded according to the PNG specifica‐
       tion.  Red, green and blue, or gray, components are gamma encoded color
       values  and  are not premultiplied by the alpha value.  The alpha value
       is a linear measure of the contribution of the pixel to the correspond‐
       ing final output pixel.

       You should normally use this format if you intend to perform color cor‐
       rection on the color values; most, maybe all, color correction software
       has  no	handling for the alpha channel and, anyway, the math to handle
       pre-multiplied component values is unnecessarily complex.

       Before you do any arithmetic on the component values you need to remove
       the  gamma  encoding  and  multiply out the alpha channel.  See the PNG
       specification for more detail.  It is important to note	that  when  an
       image  with  an alpha channel is scaled, linear encoded, pre-multiplied
       component values must be used!

       The remaining modes assume you don't need to do any further color  cor‐
       rection	or  that  if  you do, your color correction software knows all
       about alpha (it probably doesn't!).  They 'associate'  the  alpha  with
       the  color  information	by storing color channel values that have been
       scaled by the alpha.  The advantage is that the color channels  can  be
       resampled  (the image can be scaled) in this form.  The disadvantage is
       that normal practice is to store linear, not  (gamma)  encoded,	values
       and  this  requires  16-bit  channels  for still images rather than the
       8-bit channels that are just about  sufficient  if  gamma  encoding  is
       used.   In  addition  all  non-transparent pixel values, including com‐
       pletely opaque ones, must be gamma encoded to produce the final	image.
       These   are  the	 'STANDARD',  'ASSOCIATED'  or	'PREMULTIPLIED'	 modes
       described below (the latter being the two common names  for  associated
       alpha  color  channels). Note that PNG files always contain non-associ‐
       ated color channels; png_set_alpha_mode() with one of the modes	causes
       the  decoder to convert the pixels to an associated form before return‐
       ing them to your application.

       Since it is not necessary to perform arithmetic on opaque color	values
       so  long	 as  they  are	not to be resampled and are in the final color
       space it is possible to optimize the handling of alpha by  storing  the
       opaque  pixels  in the PNG format (adjusted for the output color space)
       while storing partially opaque pixels in the standard, linear,  format.
       The accuracy required for standard alpha composition is relatively low,
       because the pixels are isolated, therefore typically the accuracy  loss
       in storing 8-bit linear values is acceptable.  (This is not true if the
       alpha channel is used to simulate transparency over large areas	-  use
       16  bits	 or the PNG mode in this case!)	 This is the 'OPTIMIZED' mode.
       For this mode a pixel is treated as opaque only if the alpha  value  is
       equal to the maximum value.

	   PNG_ALPHA_STANDARD:	 The  data  libpng  produces is encoded in the
       standard way assumed by most correctly written graphics software.   The
       gamma  encoding will be removed by libpng and the linear component val‐
       ues will be pre-multiplied by the alpha channel.

       With this format the final image must be re-encoded to match  the  dis‐
       play  gamma  before  the image is displayed.  If your system doesn't do
       that, yet still seems to	 perform  arithmetic  on  the  pixels  without
       decoding them, it is broken - check out the modes below.

       With PNG_ALPHA_STANDARD libpng always produces linear component values,
       whatever screen_gamma you supply.  The screen_gamma value is,  however,
       used  as	 a  default  for  the  file gamma if the PNG file has no gamma
       information.

       If you call png_set_gamma() after png_set_alpha_mode() you  will	 over‐
       ride the linear encoding.  Instead the pre-multiplied pixel values will
       be gamma encoded but the alpha channel will still be linear.  This  may
       actually	 match	the  requirements  of  some broken software, but it is
       unlikely.

       While linear 8-bit data is often used it has insufficient precision for
       any  image  with a reasonable dynamic range.  To avoid problems, and if
       your software supports it, use png_set_expand_16() to force all	compo‐
       nents to 16 bits.

	   PNG_ALPHA_OPTIMIZED:	 This  mode  is the same as PNG_ALPHA_STANDARD
       except that completely opaque pixels are gamma encoded according to the
       screen_gamma  value.   Pixels  with alpha less than 1.0 will still have
       linear components.

       Use this format if you have control over your compositing software  and
       so don't do other arithmetic (such as scaling) on the data you get from
       libpng.	Your compositing software can simply copy opaque pixels to the
       output but still has linear values for the non-opaque pixels.

       In  normal  compositing,	 where the alpha channel encodes partial pixel
       coverage (as opposed to broad area translucency), the  inaccuracies  of
       the 8-bit representation of non-opaque pixels are irrelevant.

       You  can also try this format if your software is broken; it might look
       better.

	   PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however,  all	compo‐
       nent  values,  including	 the alpha channel are gamma encoded.  This is
       broken because, in practice, no implementation that  uses  this	choice
       correctly  undoes  the encoding before handling alpha composition.  Use
       this choice only if other serious errors in the	software  or  hardware
       you  use	 mandate it.  In most cases of broken software or hardware the
       bug in the final display manifests as a subtle halo  around  composited
       parts of the image.  You may not even perceive this as a halo; the com‐
       posited part of the image may simply appear  separate  from  the	 back‐
       ground, as though it had been cut out of paper and pasted on afterward.

       If  you don't have to deal with bugs in software or hardware, or if you
       can  fix	 them,	 there	 are   three   recommended   ways   of	 using
       png_set_alpha_mode():

	  png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
	      screen_gamma);

       You  can	 do  color correction on the result (libpng does not currently
       support color correction internally).  When you handle the alpha	 chan‐
       nel you need to undo the gamma encoding and multiply out the alpha.

	  png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
	      screen_gamma);
	  png_set_expand_16(png_ptr);

       If   you	  are	using	the   high   level   interface,	  don't	  call
       png_set_expand_16(); instead pass PNG_TRANSFORM_EXPAND_16 to the inter‐
       face.

       With  this  mode	 you  can't do color correction, but you can do arith‐
       metic, including composition and scaling, on the data  without  further
       processing.

	  png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
	      screen_gamma);

       You  can	 avoid	the expansion to 16-bit components with this mode, but
       you lose the ability to scale the image or perform other linear	arith‐
       metic.	All  you  can do is compose the result onto a matching output.
       Since this mode is libpng-specific you also need to write your own com‐
       position software.

       The  following  are  examples of calls to png_set_alpha_mode to achieve
       the required overall gamma correction and, where necessary, alpha  pre‐
       multiplication.

	   png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);

       This  is	 the  default libpng handling of the alpha channel - it is not
       pre-multiplied into the color components.  In addition the call	states
       that  the  output is for a sRGB system and causes all PNG files without
       gAMA chunks to be assumed to be encoded using sRGB.

	   png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);

       In this case the output is assumed to be something like an sRGB confor‐
       mant display preceeded by a power-law lookup table of power 1.45.  This
       is how early Mac systems behaved.

	   png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);

       This is the classic Jim Blinn approach and will work in academic	 envi‐
       ronments	 where everything is done by the book.	It has the shortcoming
       of assuming that input PNG data with no gamma information is  linear  -
       this  is	 unlikely  to  be correct unless the PNG files where generated
       locally.	 Most of the time the output precision will be so  low	as  to
       show significant banding in dark areas of the image.

	   png_set_expand_16(pp);
	   png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);

       This  is	 a  somewhat  more realistic Jim Blinn inspired approach.  PNG
       files are assumed to have the sRGB encoding if not marked with a	 gamma
       value  and  the	output	is always 16 bits per component.  This permits
       accurate scaling and processing of the data.  If	 you  know  that  your
       input  PNG  files  were	generated  locally  you	 might need to replace
       PNG_DEFAULT_sRGB with the correct value for your system.

	   png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);

       If you just need to composite the PNG image onto an existing background
       and if you control the code that does this you can use the optimization
       setting.	 In this case you just copy completely opaque  pixels  to  the
       output.	 For pixels that are not completely transparent (you just skip
       those) you do the composition math using png_composite  or  png_compos‐
       ite_16  below then encode the resultant 8-bit or 16-bit values to match
       the output encoding.

	   Other cases

       If neither the PNG nor  the  standard  linear  encoding	work  for  you
       because	of  the software or hardware you use then you have a big prob‐
       lem.  The PNG case will probably result in halos around the image.  The
       linear encoding will probably result in a washed out, too bright, image
       (it's actually too contrasty.)  Try the ALPHA_OPTIMIZED	mode  above  -
       this will probably substantially reduce the halos.  Alternatively try:

	   png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);

       This  option  will also reduce the halos, but there will be slight dark
       halos round the opaque parts of	the  image  where  the	background  is
       light.	In  the OPTIMIZED mode the halos will be light halos where the
       background is dark.  Take your pick - the halos are unavoidable	unless
       you  can	 get your hardware/software fixed!  (The OPTIMIZED approach is
       slightly faster.)

       When the default gamma of PNG files doesn't match the output gamma.  If
       you  have PNG files with no gamma information png_set_alpha_mode allows
       you to provide a default gamma, but it also sets the ouput gamma to the
       matching	 value.	  If you know your PNG files have a gamma that doesn't
       match  the  output  you	can  take   advantage	of   the   fact	  that
       png_set_alpha_mode  always  sets the output gamma but only sets the PNG
       default if it is not already set:

	   png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
	   png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);

       The first call sets both the default and the output gamma  values,  the
       second  call  overrides	the output gamma without changing the default.
       This is easier than achieving the same effect with png_set_gamma.   You
       must  use  PNG_ALPHA_PNG	 for  the  first  call	- internal checking in
       png_set_alpha will fire if more than one call to png_set_alpha_mode and
       png_set_background is made in the same read operation, however multiple
       calls with PNG_ALPHA_PNG are ignored.

       If you don't need, or can't handle, the	alpha  channel	you  can  call
       png_set_background() to remove it by compositing against a fixed color.
       Don't call png_set_strip_alpha() to do this - it	 will  leave  spurious
       pixel values in transparent parts of this image.

	  png_set_background(png_ptr, &background_color,
	      PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);

       The background_color is an RGB or grayscale value according to the data
       format libpng will produce for you.  Because you	 don't	yet  know  the
       format  of  the	PNG file, if you call png_set_background at this point
       you must arrange for the format produced by libpng to always have 8-bit
       or  16-bit  components  and  then store the color as an 8-bit or 16-bit
       color as appropriate.  The color contains separate gray and RGB	compo‐
       nent values, so you can let libpng produce gray or RGB output according
       to the input format, but low bit depth grayscale images must always  be
       converted  to  at  least	 8-bit	format.	  (Even	 though	 low bit depth
       grayscale images can't have an alpha channel they can have a  transpar‐
       ent color!)

       You  set	 the  transforms  you  need later, either as flags to the high
       level interface or libpng API calls for the low level  interface.   For
       reference the settings and API calls required are:

       8-bit values:
	  PNG_TRANSFORM_SCALE_16 | PNG_EXPAND
	  png_set_expand(png_ptr); png_set_scale_16(png_ptr);

	  If you must get exactly the same inaccurate results
	  produced by default in versions prior to libpng-1.5.4,
	  use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
	  instead.

       16-bit values:
	  PNG_TRANSFORM_EXPAND_16
	  png_set_expand_16(png_ptr);

       In either case palette image data will be expanded to RGB.  If you just
       want   color   data   you   can	 add   PNG_TRANSFORM_GRAY_TO_RGB    or
       png_set_gray_to_rgb(png_ptr) to the list.

       Calling	png_set_background before the PNG file header is read will not
       work prior to libpng-1.5.4.  Because the failure may  result  in	 unex‐
       pected	warnings  or  errors  it  is  therefore	 much  safer  to  call
       png_set_background after the head has been  read.   Unfortunately  this
       means  that prior to libpng-1.5.4 it cannot be used with the high level
       interface.

   The high-level read interface
       At this point there are two ways to  proceed;  through  the  high-level
       read  interface,	 or  through  a sequence of low-level read operations.
       You can use the high-level interface if (a) you are willing to read the
       entire image into memory, and (b) the input transformations you want to
       do are limited to the following set:

	   PNG_TRANSFORM_IDENTITY      No transformation
	   PNG_TRANSFORM_SCALE_16      Strip 16-bit samples to
				       8-bit accurately
	   PNG_TRANSFORM_STRIP_16      Chop 16-bit samples to
				       8-bit less accurately
	   PNG_TRANSFORM_STRIP_ALPHA   Discard the alpha channel
	   PNG_TRANSFORM_PACKING       Expand 1, 2 and 4-bit
				       samples to bytes
	   PNG_TRANSFORM_PACKSWAP      Change order of packed
				       pixels to LSB first
	   PNG_TRANSFORM_EXPAND	       Perform set_expand()
	   PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
	   PNG_TRANSFORM_SHIFT	       Normalize pixels to the
				       sBIT depth
	   PNG_TRANSFORM_BGR	       Flip RGB to BGR, RGBA
				       to BGRA
	   PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
				       to AG
	   PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
				       to transparency
	   PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
	   PNG_TRANSFORM_GRAY_TO_RGB   Expand grayscale samples
				       to RGB (or GA to RGBA)
	   PNG_TRANSFORM_EXPAND_16     Expand samples to 16 bits

       (This excludes setting a background color, doing gamma  transformation,
       quantizing, and setting filler.)	 If this is the case, simply do this:

	   png_read_png(png_ptr, info_ptr, png_transforms, NULL)

       where  png_transforms  is  an integer containing the bitwise OR of some
       set  of	 transformation	  flags.    This   call	  is   equivalent   to
       png_read_info(),	 followed  the set of transformations indicated by the
       transform mask, then png_read_image(), and finally png_read_end().

       (The final parameter of this call is not yet used.   Someday  it	 might
       point to transformation parameters required by some future input trans‐
       form.)

       You must use png_transforms and not call any png_set_transform()	 func‐
       tions when you use png_read_png().

       After  you  have called png_read_png(), you can retrieve the image data
       with

	  row_pointers = png_get_rows(png_ptr, info_ptr);

       where row_pointers is an array of pointers to the pixel data  for  each
       row:

	  png_bytep row_pointers[height];

       If you know your image size and pixel size ahead of time, you can allo‐
       cate row_pointers prior to calling png_read_png() with

	  if (height > PNG_UINT_32_MAX/(sizeof (png_byte)))
	     png_error (png_ptr,
		 "Image is too tall to process in memory");

	  if (width > PNG_UINT_32_MAX/pixel_size)
	     png_error (png_ptr,
		 "Image is too wide to process in memory");

	  row_pointers = png_malloc(png_ptr,
	      height*(sizeof (png_bytep)));

	  for (int i=0; i<height, i++)
	     row_pointers[i]=NULL;  /* security precaution */

	  for (int i=0; i<height, i++)
	     row_pointers[i]=png_malloc(png_ptr,
		 width*pixel_size);

	  png_set_rows(png_ptr, info_ptr, &row_pointers);

       Alternatively you could allocate your image in one big block and define
       row_pointers[i] to point into the proper places in your block.

       If  you	use png_set_rows(), the application is responsible for freeing
       row_pointers (and row_pointers[i], if they were separately allocated).

       If you don't allocate row_pointers ahead of time,  png_read_png()  will
       do it, and it'll be free'ed by libpng when you call png_destroy_*().

   The low-level read interface
       If you are going the low-level route, you are now ready to read all the
       file information up to the actual image data.  You do this with a  call
       to png_read_info().

	   png_read_info(png_ptr, info_ptr);

       This will process all chunks up to but not including the image data.

       This  also  copies  some	 of the data from the PNG file into the decode
       structure for use  in  later  transformations.	Important  information
       copied in is:

       1) The PNG file gamma from the gAMA chunk.  This overwrites the default
       value   provided	  by   an   earlier   call   to	   png_set_gamma    or
       png_set_alpha_mode.

       2)  Prior to libpng-1.5.4 the background color from a bKGd chunk.  This
       damages the information provided by an earlier  call  to	 png_set_back‐
       ground  resulting  in unexpected behavior.  Libpng-1.5.4 no longer does
       this.

       3) The number of significant bits in each component value.  Libpng uses
       this  to	 optimize gamma handling by reducing the internal lookup table
       sizes.

       4) The transparent color information from a tRNS chunk.	 This  can  be
       modified by a later call to png_set_tRNS.

   Querying the info structure
       Functions are used to get the information from the info_ptr once it has
       been read.  Note that these fields may  not  be	completely  filled  in
       until png_read_end() has read the chunk data following the image.

	   png_get_IHDR(png_ptr, info_ptr, &width, &height,
	      &bit_depth, &color_type, &interlace_type,
	      &compression_type, &filter_method);

	   width	  - holds the width of the image
			    in pixels (up to 2^31).

	   height	  - holds the height of the image
			    in pixels (up to 2^31).

	   bit_depth	  - holds the bit depth of one of the
			    image channels.  (valid values are
			    1, 2, 4, 8, 16 and depend also on
			    the color_type.  See also
			    significant bits (sBIT) below).

	   color_type	  - describes which color/alpha channels
				are present.
			    PNG_COLOR_TYPE_GRAY
			       (bit depths 1, 2, 4, 8, 16)
			    PNG_COLOR_TYPE_GRAY_ALPHA
			       (bit depths 8, 16)
			    PNG_COLOR_TYPE_PALETTE
			       (bit depths 1, 2, 4, 8)
			    PNG_COLOR_TYPE_RGB
			       (bit_depths 8, 16)
			    PNG_COLOR_TYPE_RGB_ALPHA
			       (bit_depths 8, 16)

			    PNG_COLOR_MASK_PALETTE
			    PNG_COLOR_MASK_COLOR
			    PNG_COLOR_MASK_ALPHA

	   interlace_type - (PNG_INTERLACE_NONE or
			    PNG_INTERLACE_ADAM7)

	   compression_type - (must be PNG_COMPRESSION_TYPE_BASE
			    for PNG 1.0)

	   filter_method  - (must be PNG_FILTER_TYPE_BASE
			    for PNG 1.0, and can also be
			    PNG_INTRAPIXEL_DIFFERENCING if
			    the PNG datastream is embedded in
			    a MNG-1.0 datastream)

	   Any or all of interlace_type, compression_type, or
	   filter_method can be NULL if you are
	   not interested in their values.

	   Note that png_get_IHDR() returns 32-bit data into
	   the application's width and height variables.
	   This is an unsafe situation if these are 16-bit
	   variables.  In such situations, the
	   png_get_image_width() and png_get_image_height()
	   functions described below are safer.

	   width	    = png_get_image_width(png_ptr,
				info_ptr);

	   height	    = png_get_image_height(png_ptr,
				info_ptr);

	   bit_depth	    = png_get_bit_depth(png_ptr,
				info_ptr);

	   color_type	    = png_get_color_type(png_ptr,
				info_ptr);

	   interlace_type   = png_get_interlace_type(png_ptr,
				info_ptr);

	   compression_type = png_get_compression_type(png_ptr,
				info_ptr);

	   filter_method    = png_get_filter_type(png_ptr,
				info_ptr);

	   channels = png_get_channels(png_ptr, info_ptr);

	   channels	  - number of channels of info for the
			    color type (valid values are 1 (GRAY,
			    PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
			    4 (RGB_ALPHA or RGB + filler byte))

	   rowbytes = png_get_rowbytes(png_ptr, info_ptr);

	   rowbytes	  - number of bytes needed to hold a row

	   signature = png_get_signature(png_ptr, info_ptr);

	   signature	  - holds the signature read from the
			    file (if any).  The data is kept in
			    the same offset it would be if the
			    whole signature were read (i.e. if an
			    application had already read in 4
			    bytes of signature before starting
			    libpng, the remaining 4 bytes would
			    be in signature[4] through signature[7]
			    (see png_set_sig_bytes())).

       These  are  also	 important,  but their validity depends on whether the
       chunk   has   been   read.    The   png_get_valid(png_ptr,    info_ptr,
       PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr, info_ptr, ...) functions
       return non-zero if the data has been read, or zero if  it  is  missing.
       The parameters to the png_get_<chunk> are set directly if they are sim‐
       ple data types, or a pointer into the info_ptr is returned for any com‐
       plex types.

       The  colorspace	data  from  gAMA, cHRM, sRGB, iCCP, and sBIT chunks is
       simply returned to give the application information about how the image
       was  encoded.   Libpng  itself only does transformations using the file
       gamma when combining semitransparent pixels with the background	color,
       and,  since libpng-1.6.0, when converting between 8-bit sRGB and 16-bit
       linear pixels within the simplified API.	 Libpng	 also  uses  the  file
       gamma  when converting RGB to gray, beginning with libpng-1.0.5, if the
       application calls png_set_rgb_to_gray()).

	   png_get_PLTE(png_ptr, info_ptr, &palette,
			    &num_palette);

	   palette	  - the palette for the file
			    (array of png_color)

	   num_palette	  - number of entries in the palette

	   png_get_gAMA(png_ptr, info_ptr, &file_gamma);
	   png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);

	   file_gamma	  - the gamma at which the file is
			    written (PNG_INFO_gAMA)

	   int_file_gamma - 100,000 times the gamma at which the
			    file is written

	   png_get_cHRM(png_ptr, info_ptr,  &white_x, &white_y, &red_x,
			    &red_y, &green_x, &green_y, &blue_x, &blue_y)
	   png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z,
			    &green_X, &green_Y, &green_Z, &blue_X, &blue_Y,
			    &blue_Z)
	   png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x,
			    &int_white_y, &int_red_x, &int_red_y,
			    &int_green_x, &int_green_y, &int_blue_x,
			    &int_blue_y)
	   png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
			    &int_red_Z, &int_green_X, &int_green_Y,
			    &int_green_Z, &int_blue_X, &int_blue_Y,
			    &int_blue_Z)

	   {white,red,green,blue}_{x,y}
			    A color space encoding specified using the
			    chromaticities of the end points and the
			    white point. (PNG_INFO_cHRM)

	   {red,green,blue}_{X,Y,Z}
			    A color space encoding specified using the
			    encoding end points - the CIE tristimulus
			    specification of the intended color of the red,
			    green and blue channels in the PNG RGB data.
			    The white point is simply the sum of the three
			    end points. (PNG_INFO_cHRM)

	   png_get_sRGB(png_ptr, info_ptr, &srgb_intent);

	   srgb_intent -    the rendering intent (PNG_INFO_sRGB)
			    The presence of the sRGB chunk
			    means that the pixel data is in the
			    sRGB color space.  This chunk also
			    implies specific values of gAMA and
			    cHRM.

	   png_get_iCCP(png_ptr, info_ptr, &name,
	      &compression_type, &profile, &proflen);

	   name		    - The profile name.

	   compression_type - The compression type; always
			      PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
			      You may give NULL to this argument to
			      ignore it.

	   profile	    - International Color Consortium color
			      profile data. May contain NULs.

	   proflen	    - length of profile data in bytes.

	   png_get_sBIT(png_ptr, info_ptr, &sig_bit);

	   sig_bit	  - the number of significant bits for
			    (PNG_INFO_sBIT) each of the gray,
			    red, green, and blue channels,
			    whichever are appropriate for the
			    given color type (png_color_16)

	   png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
			    &num_trans, &trans_color);

	   trans_alpha	  - array of alpha (transparency)
			    entries for palette (PNG_INFO_tRNS)

	   num_trans	  - number of transparent entries
			    (PNG_INFO_tRNS)

	   trans_color	  - graylevel or color sample values of
			    the single transparent color for
			    non-paletted images (PNG_INFO_tRNS)

	   png_get_hIST(png_ptr, info_ptr, &hist);
			    (PNG_INFO_hIST)

	   hist		  - histogram of palette (array of
			    png_uint_16)

	   png_get_tIME(png_ptr, info_ptr, &mod_time);

	   mod_time	  - time image was last modified
			   (PNG_VALID_tIME)

	   png_get_bKGD(png_ptr, info_ptr, &background);

	   background	  - background color (of type
			    png_color_16p) (PNG_VALID_bKGD)
			    valid 16-bit red, green and blue
			    values, regardless of color_type

	   num_comments	  = png_get_text(png_ptr, info_ptr,
			    &text_ptr, &num_text);

	   num_comments	  - number of comments

	   text_ptr	  - array of png_text holding image
			    comments

	   text_ptr[i].compression - type of compression used
			on "text" PNG_TEXT_COMPRESSION_NONE
				  PNG_TEXT_COMPRESSION_zTXt
				  PNG_ITXT_COMPRESSION_NONE
				  PNG_ITXT_COMPRESSION_zTXt

	   text_ptr[i].key   - keyword for comment.  Must contain
				1-79 characters.

	   text_ptr[i].text  - text comments for current
				keyword.  Can be empty.

	   text_ptr[i].text_length - length of text string,
			after decompression, 0 for iTXt

	   text_ptr[i].itxt_length - length of itxt string,
			after decompression, 0 for tEXt/zTXt

	   text_ptr[i].lang  - language of comment (empty
				string for unknown).

	   text_ptr[i].lang_key	 - keyword in UTF-8
				(empty string for unknown).

	   Note that the itxt_length, lang, and lang_key
	   members of the text_ptr structure only exist when the
	   library is built with iTXt chunk support.  Prior to
	   libpng-1.4.0 the library was built by default without
	   iTXt support. Also note that when iTXt is supported,
	   they contain NULL pointers when the "compression"
	   field contains PNG_TEXT_COMPRESSION_NONE or
	   PNG_TEXT_COMPRESSION_zTXt.

	   num_text	  - number of comments (same as
			    num_comments; you can put NULL here
			    to avoid the duplication)

	   Note while png_set_text() will accept text, language,
	   and translated keywords that can be NULL pointers, the
	   structure returned by png_get_text will always contain
	   regular zero-terminated C strings.  They might be
	   empty strings but they will never be NULL pointers.

	   num_spalettes = png_get_sPLT(png_ptr, info_ptr,
	      &palette_ptr);

	   num_spalettes  - number of sPLT chunks read.

	   palette_ptr	  - array of palette structures holding
			    contents of one or more sPLT chunks
			    read.

	   png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
	      &unit_type);

	   offset_x	  - positive offset from the left edge
			    of the screen (can be negative)

	   offset_y	  - positive offset from the top edge
			    of the screen (can be negative)

	   unit_type	  - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

	   png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
	      &unit_type);

	   res_x	  - pixels/unit physical resolution in
			    x direction

	   res_y	  - pixels/unit physical resolution in
			    x direction

	   unit_type	  - PNG_RESOLUTION_UNKNOWN,
			    PNG_RESOLUTION_METER

	   png_get_sCAL(png_ptr, info_ptr, &unit, &width,
	      &height)

	   unit	       - physical scale units (an integer)

	   width       - width of a pixel in physical scale units

	   height      - height of a pixel in physical scale units
			(width and height are doubles)

	   png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
	      &height)

	   unit	       - physical scale units (an integer)

	   width       - width of a pixel in physical scale units
			 (expressed as a string)

	   height      - height of a pixel in physical scale units
			(width and height are strings like "2.54")

	   num_unknown_chunks = png_get_unknown_chunks(png_ptr,
	      info_ptr, &unknowns)

	   unknowns	     - array of png_unknown_chunk
			       structures holding unknown chunks

	   unknowns[i].name  - name of unknown chunk

	   unknowns[i].data  - data of unknown chunk

	   unknowns[i].size  - size of unknown chunk's data

	   unknowns[i].location - position of chunk in file

	   The value of "i" corresponds to the order in which the
	   chunks were read from the PNG file or inserted with the
	   png_set_unknown_chunks() function.

	   The value of "location" is a bitwise "or" of

		PNG_HAVE_IHDR  (0x01)
		PNG_HAVE_PLTE  (0x02)
		PNG_AFTER_IDAT (0x08)

       The data from the pHYs chunk can be  retrieved  in  several  convenient
       forms:

	   res_x = png_get_x_pixels_per_meter(png_ptr,
	      info_ptr)

	   res_y = png_get_y_pixels_per_meter(png_ptr,
	      info_ptr)

	   res_x_and_y = png_get_pixels_per_meter(png_ptr,
	      info_ptr)

	   res_x = png_get_x_pixels_per_inch(png_ptr,
	      info_ptr)

	   res_y = png_get_y_pixels_per_inch(png_ptr,
	      info_ptr)

	   res_x_and_y = png_get_pixels_per_inch(png_ptr,
	      info_ptr)

	   aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
	      info_ptr)

	   Each of these returns 0 [signifying "unknown"] if
	      the data is not present or if res_x is 0;
	      res_x_and_y is 0 if res_x != res_y

	   Note that because of the way the resolutions are
	      stored internally, the inch conversions won't
	      come out to exactly even number.	For example,
	      72 dpi is stored as 0.28346 pixels/meter, and
	      when this is retrieved it is 71.9988 dpi, so
	      be sure to round the returned value appropriately
	      if you want to display a reasonable-looking result.

       The  data  from	the  oFFs chunk can be retrieved in several convenient
       forms:

	   x_offset = png_get_x_offset_microns(png_ptr, info_ptr);

	   y_offset = png_get_y_offset_microns(png_ptr, info_ptr);

	   x_offset = png_get_x_offset_inches(png_ptr, info_ptr);

	   y_offset = png_get_y_offset_inches(png_ptr, info_ptr);

	   Each of these returns 0 [signifying "unknown" if both
	      x and y are 0] if the data is not present or if the
	      chunk is present but the unit is the pixel.  The
	      remark about inexact inch conversions applies here
	      as well, because a value in inches can't always be
	      converted to microns and back without some loss
	      of precision.

       For more information, see the PNG specification for chunk contents.  Be
       careful	with  trusting	rowbytes, as some of the transformations could
       increase the space needed to hold a row (expand,	 filler,  gray_to_rgb,
       etc.).  See png_read_update_info(), below.

       A  quick word about text_ptr and num_text.  PNG stores comments in key‐
       word/text pairs, one pair per chunk, with no limit  on  the  number  of
       text chunks, and a 2^31 byte limit on their size.  While there are sug‐
       gested keywords, there is no requirement to restrict the use  to	 these
       strings.	  It  is strongly suggested that keywords and text be sensible
       to humans (that's the point), so don't use abbreviations.  Non-printing
       symbols	are  not allowed.  See the PNG specification for more details.
       There is also no requirement to have text after the keyword.

       Keywords should be limited to 79 Latin-1 characters without leading  or
       trailing spaces, but non-consecutive spaces are allowed within the key‐
       word.  It is possible to have the same keyword  any  number  of	times.
       The text_ptr is an array of png_text structures, each holding a pointer
       to a language string, a pointer to a keyword and a pointer  to  a  text
       string.	 The text string, language code, and translated keyword may be
       empty or NULL pointers.	The keyword/text pairs are put into the	 array
       in  the order that they are received.  However, some or all of the text
       chunks may be after the image, so, to make sure you have read  all  the
       text chunks, don't mess with these until after you read the stuff after
       the image.  This will be mentioned again below in the  discussion  that
       goes with png_read_end().

   Input transformations
       After you've read the header information, you can set up the library to
       handle any special transformations of the image data.  The various ways
       to  transform  the data will be described in the order that they should
       occur.  This is important, as some  of  these  change  the  color  type
       and/or  bit  depth  of  the  data, and some others only work on certain
       color types and bit depths.

       Transformations you request are ignored if they don't have any  meaning
       for  a  particular input data format.  However some transformations can
       have an effect as a result of a previous transformation.	 If you	 spec‐
       ify a contradictory set of transformations, for example both adding and
       removing the alpha channel, you cannot predict the final result.

       The color used for the transparency values should be  supplied  in  the
       same  format/depth as the current image data.  It is stored in the same
       format/depth as the image data in a tRNS chunk, so this is what	libpng
       expects for this data.

       The  color  used	 for  the  background value depends on the need_expand
       argument as described below.

       Data will be decoded into the supplied row buffers  packed  into	 bytes
       unless  the  library has been told to transform it into another format.
       For example, 4 bit/pixel paletted or grayscale data will be returned  2
       pixels/byte with the leftmost pixel in the high-order bits of the byte,
       unless png_set_packing() is called.  8-bit RGB data will be  stored  in
       RGB  RGB	 RGB  format unless png_set_filler() or png_set_add_alpha() is
       called to insert filler bytes, either before or after each RGB triplet.
       16-bit  RGB data will be returned RRGGBB RRGGBB, with the most signifi‐
       cant byte of the color value first, unless png_set_scale_16() is called
       to  transform  it  to  regular RGB RGB triplets, or png_set_filler() or
       png_set_add alpha() is called to insert filler bytes, either before  or
       after  each  RRGGBB triplet.  Similarly, 8-bit or 16-bit grayscale data
       can   be	  modified   with    png_set_filler(),	  png_set_add_alpha(),
       png_set_strip_16(), or png_set_scale_16().

       The  following  code  transforms	 grayscale  images of less than 8 to 8
       bits, changes paletted images to RGB, and adds a full alpha channel  if
       there is transparency information in a tRNS chunk.  This is most useful
       on grayscale images with bit depths of 2 or 4 or if there is  a	multi‐
       ple-image  viewing  application	that wishes to treat all images in the
       same way.

	   if (color_type == PNG_COLOR_TYPE_PALETTE)
	       png_set_palette_to_rgb(png_ptr);

	   if (png_get_valid(png_ptr, info_ptr,
	       PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);

	   if (color_type == PNG_COLOR_TYPE_GRAY &&
	       bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);

       The first two functions	are  actually  aliases	for  png_set_expand(),
       added  in  libpng  version  1.0.4,  with the function names expanded to
       improve code readability.  In some future version they may actually  do
       different things.

       As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was added.
       It expands the sample depth without changing tRNS to alpha.

       As of libpng version 1.5.2, png_set_expand_16() was added.  It  behaves
       as  png_set_expand();  however,	the  resultant	channels  have 16 bits
       rather than 8.  Use this when the output color  or  gray	 channels  are
       made linear to avoid fairly severe accuracy loss.

	  if (bit_depth < 16)
	     png_set_expand_16(png_ptr);

       PNG  can have files with 16 bits per channel.  If you only can handle 8
       bits per channel, this will strip the pixels down to 8-bit.

	   if (bit_depth == 16) #if PNG_LIBPNG_VER >= 10504
	      png_set_scale_16(png_ptr); #else
	      png_set_strip_16(png_ptr); #endif

       (The more accurate "png_set_scale_16()" API became available in	libpng
       version 1.5.4).

       If  you	need to process the alpha channel on the image separately from
       the image data (for example if you convert it to a bitmap mask)	it  is
       possible	 to  have  libpng  strip  the channel leaving just RGB or gray
       data:

	   if (color_type & PNG_COLOR_MASK_ALPHA)
	      png_set_strip_alpha(png_ptr);

       If you strip the alpha channel you need to find some other way of deal‐
       ing  with  the information.  If, instead, you want to convert the image
       to an opaque version with no alpha channel use png_set_background;  see
       below.

       As of libpng version 1.5.2, almost all useful expansions are supported,
       the major ommissions are conversion  of	grayscale  to  indexed	images
       (which  can  be	done  trivially	 in the application) and conversion of
       indexed to grayscale (which can be done by a  trivial  manipulation  of
       the palette.)

       In  the	following table, the 01 means grayscale with depth<8, 31 means
       indexed with depth<8, other numerals  represent	the  color  type,  "T"
       means  the  tRNS chunk is present, A means an alpha channel is present,
       and O means tRNS or alpha is present but all pixels in  the  image  are
       opaque.

	 FROM  01  31	0  0T  0O   2  2T  2O	3  3T  3O  4A  4O  6A  6O
	  TO
	  01	-  [G]	-   -	-   -	-   -	-   -	-   -	-   -	-
	  31   [Q]  Q  [Q] [Q] [Q]  Q	Q   Q	Q   Q	Q  [Q] [Q]  Q	Q
	   0	1   G	+   .	.   G	G   G	G   G	G   B	B  GB  GB
	  0T	lt  Gt	t   +	.   Gt	G   G	Gt  G	G   Bt	Bt GBt GBt
	  0O	lt  Gt	t   .	+   Gt	Gt  G	Gt  Gt	G   Bt	Bt GBt GBt
	   2	C   P	C   C	C   +	.   .	C   -	-  CB  CB   B	B
	  2T	Ct  -	Ct  C	C   t	+   t	-   -	-  CBt CBt  Bt	Bt
	  2O	Ct  -	Ct  C	C   t	t   +	-   -	-  CBt CBt  Bt	Bt
	   3   [Q]  p  [Q] [Q] [Q]  Q	Q   Q	+   .	.  [Q] [Q]  Q	Q
	  3T   [Qt] p  [Qt][Q] [Q]  Qt	Qt  Qt	t   +	t  [Qt][Qt] Qt	Qt
	  3O   [Qt] p  [Qt][Q] [Q]  Qt	Qt  Qt	t   t	+  [Qt][Qt] Qt	Qt
	  4A	lA  G	A   T	T   GA	GT  GT	GA  GT	GT  +	BA  G  GBA
	  4O	lA GBA	A   T	T   GA	GT  GT	GA  GT	GT  BA	+  GBA	G
	  6A	CA  PA	CA  C	C   A	T  tT	PA  P	P   C  CBA  +	BA
	  6O	CA PBA	CA  C	C   A  tT   T	PA  P	P  CBA	C   BA	+

       Within the matrix,
	    "+" identifies entries where 'from' and 'to' are the same.
	    "-" means the transformation is not supported.
	    "." means nothing is necessary (a tRNS chunk can just be ignored).
	    "t" means the transformation is obtained by png_set_tRNS.
	    "A" means the transformation is obtained by png_set_add_alpha().
	    "X" means the transformation is obtained by png_set_expand().
	    "1" means the transformation is obtained by
		png_set_expand_gray_1_2_4_to_8() (and by png_set_expand()
		if there is no transparency in the original or the final
		format).
	    "C" means the transformation is obtained by png_set_gray_to_rgb().
	    "G" means the transformation is obtained by png_set_rgb_to_gray().
	    "P" means the transformation is obtained by
		png_set_expand_palette_to_rgb().
	    "p" means the transformation is obtained by png_set_packing().
	    "Q" means the transformation is obtained by png_set_quantize().
	    "T" means the transformation is obtained by
		png_set_tRNS_to_alpha().
	    "B" means the transformation is obtained by
		png_set_background(), or png_strip_alpha().

       When  an entry has multiple transforms listed all are required to cause
       the right overall transformation.  When two transforms are separated by
       a comma either will do the job.	When transforms are enclosed in [] the
       transform should do the job but this is	currently  unimplemented  -  a
       different format will result if the suggested transformations are used.

       In  PNG	files,	the alpha channel in an image is the level of opacity.
       If you need the alpha channel in an image to be	the  level  of	trans‐
       parency	instead	 of  opacity, you can invert the alpha channel (or the
       tRNS chunk data) after it's read, so that 0 is fully opaque and 255 (in
       8-bit  or  paletted images) or 65535 (in 16-bit images) is fully trans‐
       parent, with

	   png_set_invert_alpha(png_ptr);

       PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small  as
       they can, resulting in, for example, 8 pixels per byte for 1 bit files.
       This code expands to 1 pixel per byte without changing  the  values  of
       the pixels:

	   if (bit_depth < 8)
	      png_set_packing(png_ptr);

       PNG  files  have possible bit depths of 1, 2, 4, 8, and 16.  All pixels
       stored in a PNG image have been "scaled" or "shifted" up	 to  the  next
       higher  possible bit depth (e.g. from 5 bits/sample in the range [0,31]
       to 8 bits/sample in the range [0, 255]).	 However, it is also  possible
       to  convert  the	 PNG  pixel data back to the original bit depth of the
       image.  This call reduces the pixels back  down	to  the	 original  bit
       depth:

	   png_color_8p sig_bit;

	   if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
	      png_set_shift(png_ptr, sig_bit);

       PNG  files  store  3-color pixels in red, green, blue order.  This code
       changes the storage of the pixels to blue, green, red:

	   if (color_type == PNG_COLOR_TYPE_RGB ||
	       color_type == PNG_COLOR_TYPE_RGB_ALPHA)
	      png_set_bgr(png_ptr);

       PNG files store RGB pixels packed into 3 or 6 bytes. This code  expands
       them  into  4  or  8 bytes for windowing systems that need them in this
       format:

	   if (color_type == PNG_COLOR_TYPE_RGB)
	      png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);

       where "filler" is the 8 or 16-bit number to fill with, and the location
       is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
       you want the filler before the RGB or after.  This transformation  does
       not  affect  images  that  already have full alpha channels.  To add an
       opaque alpha channel, use filler=0xff or	 0xffff	 and  PNG_FILLER_AFTER
       which will generate RGBA pixels.

       Note that png_set_filler() does not change the color type.  If you want
       to do that, you can add a true alpha channel with

	   if (color_type == PNG_COLOR_TYPE_RGB ||
	      color_type == PNG_COLOR_TYPE_GRAY)
	      png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);

       where "filler" contains the alpha value to assign to each pixel.	  This
       function was added in libpng-1.2.7.

       If  you	are  reading  an image with an alpha channel, and you need the
       data as ARGB instead of the normal PNG format RGBA:

	   if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
	      png_set_swap_alpha(png_ptr);

       For some uses, you may want a grayscale image to be represented as RGB.
       This code will do that conversion:

	   if (color_type == PNG_COLOR_TYPE_GRAY ||
	       color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
	      png_set_gray_to_rgb(png_ptr);

       Conversely,  you	 can  convert  an  RGB	or  RGBA image to grayscale or
       grayscale with alpha.

	   if (color_type == PNG_COLOR_TYPE_RGB ||
	       color_type == PNG_COLOR_TYPE_RGB_ALPHA)
	      png_set_rgb_to_gray(png_ptr, error_action,
		 double red_weight, double green_weight);

	   error_action = 1: silently do the conversion

	   error_action = 2: issue a warning if the original
			     image has any pixel where
			     red != green or red != blue

	   error_action = 3: issue an error and abort the
			     conversion if the original
			     image has any pixel where
			     red != green or red != blue

	   red_weight:	     weight of red component

	   green_weight:     weight of green component
			     If either weight is negative, default
			     weights are used.

       In the corresponding fixed point API the	 red_weight  and  green_weight
       values are simply scaled by 100,000:

	   png_set_rgb_to_gray(png_ptr, error_action,
	      png_fixed_point red_weight,
	      png_fixed_point green_weight);

       If  you have set error_action = 1 or 2, you can later check whether the
       image really was gray,  after  processing  the  image  rows,  with  the
       png_get_rgb_to_gray_status(png_ptr)   function.	  It   will  return  a
       png_byte that is zero if the image was gray or 1 if there were any non-
       gray  pixels.   Background  and sBIT data will be silently converted to
       grayscale, using the green channel data for  sBIT,  regardless  of  the
       error_action setting.

       The default values come from the PNG file cHRM chunk if present; other‐
       wise, the defaults correspond to the ITU-R recommendation 709, and also
       the  sRGB  color	 space, as recommended in the Charles Poynton's Colour
       FAQ, <http://www.poynton.com/>, in section 9:

	  <http://www.poynton.com/notes/colour_and_gamma/Color‐
       FAQ.html#RTFToC9>

	   Y = 0.2126 * R + 0.7152 * G + 0.0722 * B

       Previous	 versions  of  this document, 1998 through 2002, recommended a
       slightly different formula:

	   Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

       Libpng uses an integer approximation:

	   Y = (6968 * R + 23434 * G + 2366 * B)/32768

       The calculation is done in a linear colorspace, if the image gamma  can
       be determined.

       The  png_set_background() function has been described already; it tells
       libpng to composite images with alpha or	 simple	 transparency  against
       the  supplied  background  color.   For	compatibility with versions of
       libpng earlier than libpng-1.5.4 it is recommended that	you  call  the
       function	 after	reading the file header, even if you don't want to use
       the color in a bKGD chunk, if one exists.

       If the PNG file contains a bKGD chunk (PNG_INFO_bKGD  valid),  you  may
       use  this  color, or supply another color more suitable for the current
       display (e.g., the background color from a web page).  You need to tell
       libpng  how  the color is represented, both the format of the component
       values in the color (the number of bits) and the gamma encoding of  the
       color.	The  function  takes  two arguments, background_gamma_mode and
       need_expand to convey this information; however, only two  combinations
       are likely to be useful:

	   png_color_16 my_background;
	   png_color_16p image_background;

	   if (png_get_bKGD(png_ptr, info_ptr, &image_background))
	      png_set_background(png_ptr, image_background,
		  PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
	   else
	      png_set_background(png_ptr, &my_background,
		  PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);

       The second call was described above - my_background is in the format of
       the final, display, output produced by libpng.  Because	you  now  know
       the format of the PNG it is possible to avoid the need to choose either
       8-bit or 16-bit output and to retain palette images (the palette colors
       will  be	 modified appropriately and the tRNS chunk removed.)  However,
       if you are doing this, take great care not to ask  for  transformations
       without checking first that they apply!

       In  the	first call the background color has the original bit depth and
       color type of the PNG file.  So, for palette images the color  is  sup‐
       plied  as a palette index and for low bit greyscale images the color is
       a reduced bit value in image_background->gray.

       If you didn't call png_set_gamma() before reading the file header,  for
       example	if you need your code to remain compatible with older versions
       of libpng prior to libpng-1.5.4, this is the place to call it.

       Do not call it if you called png_set_alpha_mode(); doing so will damage
       the    settings	  put	in   place   by	  png_set_alpha_mode().	   (If
       png_set_alpha_mode()  is	 supported   then   you	  can	certainly   do
       png_set_gamma() before reading the PNG header.)

       This  API  unconditionally sets the screen and file gamma values, so it
       will override the value in the PNG file unless it is called before  the
       PNG  file reading starts.  For this reason you must always call it with
       the PNG file value when you call it in this position:

	  if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
	     png_set_gamma(png_ptr, screen_gamma, file_gamma);

	  else
	     png_set_gamma(png_ptr, screen_gamma, 0.45455);

       If you need to reduce an RGB file to a paletted file, or if a  paletted
       file  has more entries then will fit on your screen, png_set_quantize()
       will do that.  Note that this  is  a  simple  match  quantization  that
       merely finds the closest color available.  This should work fairly well
       with optimized palettes, but fairly badly with linear color cubes.   If
       you  pass  a  palette that is larger than maximum_colors, the file will
       reduce the number of colors in the palette so it will  fit  into	 maxi‐
       mum_colors.   If	 there is a histogram, libpng will use it to make more
       intelligent choices when reducing the palette.  If  there  is  no  his‐
       togram, it may not do as good a job.

	  if (color_type & PNG_COLOR_MASK_COLOR)
	  {
	     if (png_get_valid(png_ptr, info_ptr,
		 PNG_INFO_PLTE))
	     {
		png_uint_16p histogram = NULL;

		png_get_hIST(png_ptr, info_ptr,
		    &histogram);
		png_set_quantize(png_ptr, palette, num_palette,
		   max_screen_colors, histogram, 1);
	     }

	     else
	     {
		png_color std_color_cube[MAX_SCREEN_COLORS] =
		   { ... colors ... };

		png_set_quantize(png_ptr, std_color_cube,
		   MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
		   NULL,0);
	     }
	  }

       PNG  files describe monochrome as black being zero and white being one.
       The following code will reverse this (make black be one	and  white  be
       zero):

	  if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
	     png_set_invert_mono(png_ptr);

       This  function  can  also  be  used  to invert grayscale and gray-alpha
       images:

	  if (color_type == PNG_COLOR_TYPE_GRAY ||
	      color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
	     png_set_invert_mono(png_ptr);

       PNG files store 16-bit pixels in network byte  order  (big-endian,  ie.
       most  significant  bits	first).	  This code changes the storage to the
       other way (little-endian, i.e. least significant bits  first,  the  way
       PCs store them):

	   if (bit_depth == 16)
	      png_set_swap(png_ptr);

       If  you	are using packed-pixel images (1, 2, or 4 bits/pixel), and you
       need to change the order the pixels are packed into bytes, you can use:

	   if (bit_depth < 8)
	      png_set_packswap(png_ptr);

       Finally, you can write your own transformation function if none of  the
       existing	 ones  meets  your  needs.  This is done by setting a callback
       with

	   png_set_read_user_transform_fn(png_ptr,
	       read_transform_fn);

       You must supply the function

	   void read_transform_fn(png_structp png_ptr, png_row_infop
	       row_info, png_bytep data)

       See pngtest.c for a working example.   Your  function  will  be	called
       after  all of the other transformations have been processed.  Take care
       with interlaced images if you do the interlace yourself - the width  of
       the row is the width in 'row_info', not the overall image width.

       If supported, libpng provides two information routines that you can use
       to find where you are in processing the image:

	  png_get_current_pass_number(png_structp png_ptr);
	  png_get_current_row_number(png_structp png_ptr);

       Don't try using these outside a transform callback - firstly  they  are
       only supported if user transforms are supported, secondly they may well
       return unexpected results unless the row is actually being processed at
       the moment they are called.

       With  interlaced images the value returned is the row in the input sub-
       image	image.	   Use	  PNG_ROW_FROM_PASS_ROW(row,	 pass)	   and
       PNG_COL_FROM_PASS_COL(col,  pass)  to find the output pixel (x,y) given
       an interlaced sub-image pixel (row,col,pass).

       The discussion of interlace handling above contains more information on
       how to use these values.

       You can also set up a pointer to a user structure for use by your call‐
       back function, and you can inform libpng that your  transform  function
       will change the number of channels or bit depth with the function

	   png_set_user_transform_info(png_ptr, user_ptr,
	       user_depth, user_channels);

       The  user's  application, not libpng, is responsible for allocating and
       freeing any memory required for the user structure.

       You can retrieve	 the  pointer  via  the	 function  png_get_user_trans‐
       form_ptr().  For example:

	   voidp read_user_transform_ptr =
	       png_get_user_transform_ptr(png_ptr);

       The  last  thing	 to  handle  is interlacing; this is covered in detail
       below, but you must call the function here if you want libpng to handle
       expansion of the interlaced image.

	   number_of_passes = png_set_interlace_handling(png_ptr);

       After  setting  the  transformations,  libpng  can update your png_info
       structure to reflect any transformations	 you've	 requested  with  this
       call.

	   png_read_update_info(png_ptr, info_ptr);

       This  is	 most  useful to update the info structure's rowbytes field so
       you can use it to allocate your image memory.  This function will  also
       update  your  palette  with  the correct screen_gamma and background if
       these have been	given  with  the  calls	 above.	  You  may  only  call
       png_read_update_info() once with a particular info_ptr.

       After  you call png_read_update_info(), you can allocate any memory you
       need to hold the image.	The row data is simply raw byte data  for  all
       forms  of  images.  As the actual allocation varies among applications,
       no example will be given.  If you are allocating one large  chunk,  you
       will  need  to  build  an  array of pointers to each row, as it will be
       needed for some of the functions below.

       Remember: Before you call png_read_update_info(), the png_get_*() func‐
       tions return the values corresponding to the original PNG image.	 After
       you call png_read_update_info the values refer to the image that libpng
       will  output.   Consequently  you  must call all the png_set_ functions
       before you call png_read_update_info().	This is particularly important
       for   png_set_interlace_handling()   -	if   you  are  going  to  call
       png_read_update_info()  you  must   call	  png_set_interlace_handling()
       before it unless you want to receive interlaced output.

   Reading image data
       After  you've  allocated memory, you can read the image data.  The sim‐
       plest way to do this is in one function call.  If  you  are  allocating
       enough	memory	 to   hold   the   whole  image,  you  can  just  call
       png_read_image() and libpng will read in all the image data and put  it
       in  the	memory	area  supplied.	  You will need to pass in an array of
       pointers to each row.

       This function automatically handles interlacing, so you don't  need  to
       call	  png_set_interlace_handling()	    (unless	 you	  call
       png_read_update_info()) or call this function multiple times, or any of
       that other stuff necessary with png_read_rows().

	  png_read_image(png_ptr, row_pointers);

       where row_pointers is:

	  png_bytep row_pointers[height];

       You can point to void or char or whatever you use for pixels.

       If  you	don't  want  to	 read  in the whole image at once, you can use
       png_read_rows() instead.	 If there  is  no  interlacing	(check	inter‐
       lace_type == PNG_INTERLACE_NONE), this is simple:

	   png_read_rows(png_ptr, row_pointers, NULL,
	       number_of_rows);

       where row_pointers is the same as in the png_read_image() call.

       If  you	are  doing this just one row at a time, you can do this with a
       single row_pointer instead of an array of row_pointers:

	   png_bytep row_pointer = row;
	   png_read_row(png_ptr, row_pointer, NULL);

       If the file is interlaced (interlace_type !=  0	in  the	 IHDR  chunk),
       things  get  somewhat harder.  The only current (PNG Specification ver‐
       sion 1.2) interlacing type for PNG  is  (interlace_type	==  PNG_INTER‐
       LACE_ADAM7);  a	somewhat  complicated  2D  interlace  scheme, known as
       Adam7, that breaks down an image into seven smaller images  of  varying
       size,  based  on an 8x8 grid.  This number is defined (from libpng 1.5)
       as PNG_INTERLACE_ADAM7_PASSES in png.h

       libpng can fill out those images or it can give them to	you  "as  is".
       It  is  almost  always better to have libpng handle the interlacing for
       you.  If you want the images filled out, there are two ways to do that.
       The  one	 mentioned in the PNG specification is to expand each pixel to
       cover those pixels  that	 have  not  been  read	yet  (the  "rectangle"
       method).	  This	results	 in  a	blocky image for the first pass, which
       gradually smooths out as more pixels are read.  The other method is the
       "sparkle" method, where pixels are drawn only in their final locations,
       with the rest of the image remaining whatever colors they were initial‐
       ized  to	 before the start of the read.	The first method usually looks
       better, but tends to be slower, as there are more pixels to put in  the
       rows.

       If,  as	is  likely,  you  want	libpng to expand the images, call this
       before calling png_start_read_image() or png_read_update_info():

	   if (interlace_type == PNG_INTERLACE_ADAM7)
	      number_of_passes
		  = png_set_interlace_handling(png_ptr);

       This will return the number  of	passes	needed.	  Currently,  this  is
       seven,  but  may change if another interlace type is added.  This func‐
       tion can be called even if the file is not interlaced,  where  it  will
       return  one  pass.   You	 then  need  to	 read  the  whole  image 'num‐
       ber_of_passes' times.  Each time will distribute the  pixels  from  the
       current	pass  to the correct place in the output image, so you need to
       supply the same rows to png_read_rows in each pass.

       If you are not going to display the image  after	 each  pass,  but  are
       going  to  wait	until  the  entire  image  is read in, use the sparkle
       effect.	This effect is faster and the end result of either  method  is
       exactly	the  same.   If you are planning on displaying the image after
       each pass, the "rectangle" effect is generally  considered  the	better
       looking one.

       If  you	only  want  the "sparkle" effect, just call png_read_rows() as
       normal, with the third parameter NULL.  Make sure you  make  pass  over
       the  image number_of_passes times, and you don't change the data in the
       rows between calls.  You can change the locations of the data, just not
       the  data.  Each pass only writes the pixels appropriate for that pass,
       and assumes the data from previous passes is still valid.

	   png_read_rows(png_ptr, row_pointers, NULL,
	       number_of_rows);

       If you only want the first effect (the  rectangles),  do	 the  same  as
       before except pass the row buffer in the third parameter, and leave the
       second parameter NULL.

	   png_read_rows(png_ptr, NULL, row_pointers,
	       number_of_rows);

       If you don't want libpng to handle the interlacing details,  just  call
       png_read_rows()	PNG_INTERLACE_ADAM7_PASSES  times  to  read in all the
       images.	Each of the images is a valid image by	itself;	 however,  you
       will almost certainly need to distribute the pixels from each sub-image
       to the correct place.  This is where everything gets very tricky.

       If you want to retrieve the separate images you must pass  the  correct
       number  of rows to each successive call of png_read_rows().  The calcu‐
       lation gets pretty complicated for small images, where some  sub-images
       may  not	 even exist because either their width or height ends up zero.
       libpng provides two macros to help you in 1.5 and later versions:

	  png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
	  png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);

       Respectively these tell you the width and height of the sub-image  cor‐
       responding  to  the  numbered pass.  'pass' is in in the range 0 to 6 -
       this can be confusing because the  specification	 refers	 to  the  same
       passes as 1 to 7!  Be careful, you must check both the width and height
       before calling png_read_rows() and not call it for that pass if	either
       is zero.

       You  can,  of  course,  read each sub-image row by row.	If you want to
       produce optimal code to make  a	pixel-by-pixel	transformation	of  an
       interlaced image this is the best approach; read each row of each pass,
       transform it, and write it out to a new interlaced image.

       If you want to de-interlace the image yourself libpng provides  further
       macros  to  help	 that tell you where to place the pixels in the output
       image.  Because the interlacing scheme is rectangular - sub-image  pix‐
       els  are	 always	 arranged on a rectangular grid - all you need to know
       for each pass is the starting column and row in the output image of the
       first  pixel  plus  the	spacing	 between each pixel.  As of libpng 1.5
       there are four macros to retrieve this information:

	  png_uint_32 x = PNG_PASS_START_COL(pass);
	  png_uint_32 y = PNG_PASS_START_ROW(pass);
	  png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
	  png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);

       These allow you to write the obvious loop:

	  png_uint_32 input_y = 0;
	  png_uint_32 output_y = PNG_PASS_START_ROW(pass);

	  while (output_y < output_image_height)
	  {
	     png_uint_32 input_x = 0;
	     png_uint_32 output_x = PNG_PASS_START_COL(pass);

	     while (output_x < output_image_width)
	     {
		image[output_y][output_x] =
		    subimage[pass][input_y][input_x++];

		output_x += xStep;
	     }

	     ++input_y;
	     output_y += yStep;
	  }

       Notice that the steps between successive output rows  and  columns  are
       returned	 as shifts.  This is possible because the pixels in the subim‐
       ages are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the orig‐
       inal  image.  In practice you may need to directly calculate the output
       coordinate given an input  coordinate.	libpng	provides  two  further
       macros for this purpose:

	  png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
	  png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);

       Finally a pair of macros are provided to tell you if a particular image
       row or column appears in a given pass:

	  int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
	  int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);

       Bear in mind that you will probably also need to check  the  width  and
       height  of  the	pass in addition to the above to be sure the pass even
       exists!

       With any luck you are convinced by now that you don't want to  do  your
       own  interlace  handling.  In reality normally the only good reason for
       doing this is if you are processing PNG files on a pixel-by-pixel basis
       and  don't  want	 to  load the whole file into memory when it is inter‐
       laced.

       libpng includes a test program, pngvalid, that illustrates reading  and
       writing	of interlaced images.  If you can't get interlacing to work in
       your code and don't  want  to  leave  it	 to  libpng  (the  recommended
       approach), see how pngvalid.c does it.

   Finishing a sequential read
       After  you  are finished reading the image through the low-level inter‐
       face, you can finish reading the file.

       If you want to use a different crc action for handling  CRC  errors  in
       chunks after the image data, you can call png_set_crc_action() again at
       this point.

       If you are interested in comments or time, which may be	stored	either
       before  or  after the image data, you should pass the separate png_info
       struct if you want to keep the comments from before and after the image
       separate.

	   png_infop end_info = png_create_info_struct(png_ptr);

	   if (!end_info)
	   {
	      png_destroy_read_struct(&png_ptr, &info_ptr,
		  (png_infopp)NULL);
	      return (ERROR);
	   }

	  png_read_end(png_ptr, end_info);

       If you are not interested, you should still call png_read_end() but you
       can pass NULL, avoiding the need to create an end_info  structure.   If
       you  do	this, libpng will not process any chunks after IDAT other than
       skipping over them and perhaps (depending on whether  you  have	called
       png_set_crc_action)  checking  their  CRCs  while  looking for the IEND
       chunk.

	  png_read_end(png_ptr, (png_infop)NULL);

       If you don't call png_read_end(), then your file pointer will  be  left
       pointing	 to the first chunk after the last IDAT, which is probably not
       what you want if you expect to read something beyond the end of the PNG
       datastream.

       When  you  are  done,  you can free all memory allocated by libpng like
       this:

	  png_destroy_read_struct(&png_ptr, &info_ptr,
	      &end_info);

       or, if you didn't create an end_info structure,

	  png_destroy_read_struct(&png_ptr, &info_ptr,
	      (png_infopp)NULL);

       It is also possible to individually  free  the  info_ptr	 members  that
       point to libpng-allocated storage with the following function:

	   png_free_data(png_ptr, info_ptr, mask, seq)

	   mask - identifies data to be freed, a mask
		  containing the bitwise OR of one or
		  more of
		    PNG_FREE_PLTE, PNG_FREE_TRNS,
		    PNG_FREE_HIST, PNG_FREE_ICCP,
		    PNG_FREE_PCAL, PNG_FREE_ROWS,
		    PNG_FREE_SCAL, PNG_FREE_SPLT,
		    PNG_FREE_TEXT, PNG_FREE_UNKN,
		  or simply PNG_FREE_ALL

	   seq	- sequence number of item to be freed
		  (-1 for all items)

       This  function  may  be	safely	called	when  the relevant storage has
       already been freed, or has not yet been allocated, or was allocated  by
       the  user  and not by libpng,  and will in those cases do nothing.  The
       "seq" parameter is ignored if only one item of the selected data	 type,
       such  as	 PLTE, is allowed.  If "seq" is not -1, and multiple items are
       allowed for the data type identified in the mask, such as text or sPLT,
       only the n'th item in the structure is freed, where n is "seq".

       The default behavior is only to free data that was allocated internally
       by libpng.  This can be changed, so that libpng will not free the data,
       or  so  that  it	 will  free  data  that was allocated by the user with
       png_malloc() or png_calloc() and passed in via a png_set_*()  function,
       with

	   png_data_freer(png_ptr, info_ptr, freer, mask)

	   freer  - one of
		      PNG_DESTROY_WILL_FREE_DATA
		      PNG_SET_WILL_FREE_DATA
		      PNG_USER_WILL_FREE_DATA

	   mask	  - which data elements are affected
		    same choices as in png_free_data()

       This  function  only affects data that has already been allocated.  You
       can call this function after reading the PNG data  but  before  calling
       any   png_set_*()  functions,  to  control  whether  the	 user  or  the
       png_set_*() function is responsible for freeing any existing data  that
       might  be present, and again after the png_set_*() functions to control
       whether the user or png_destroy_*() is supposed to free the data.  When
       the user assumes responsibility for libpng-allocated data, the applica‐
       tion must use png_free() to  free  it,  and  when  the  user  transfers
       responsibility to libpng for data that the user has allocated, the user
       must have used png_malloc() or png_calloc() to allocate it.

       If you allocated your row_pointers in  a	 single	 block,	 as  suggested
       above in the description of the high level read interface, you must not
       transfer	 responsibility	 for  freeing  it  to  the   png_set_rows   or
       png_read_destroy	 function,  because  they  would  also try to free the
       individual row_pointers[i].

       If you  allocated  text_ptr.text,  text_ptr.lang,  and  text_ptr.trans‐
       lated_keyword  separately,  do  not transfer responsibility for freeing
       text_ptr to libpng, because when libpng fills a png_text	 structure  it
       combines	 these	members	 with the key member, and png_free_data() will
       free only text_ptr.key.	Similarly, if you transfer responsibility  for
       free'ing	 text_ptr  from	 libpng	 to your application, your application
       must not separately free those members.

       The png_free_data() function will turn off the "valid"  flag  for  any‐
       thing  it frees.	 If you need to turn the flag off for a chunk that was
       freed by your application instead of by libpng, you can use

	   png_set_invalid(png_ptr, info_ptr, mask);

	   mask - identifies the chunks to be made invalid,
		  containing the bitwise OR of one or
		  more of
		    PNG_INFO_gAMA, PNG_INFO_sBIT,
		    PNG_INFO_cHRM, PNG_INFO_PLTE,
		    PNG_INFO_tRNS, PNG_INFO_bKGD,
		    PNG_INFO_hIST, PNG_INFO_pHYs,
		    PNG_INFO_oFFs, PNG_INFO_tIME,
		    PNG_INFO_pCAL, PNG_INFO_sRGB,
		    PNG_INFO_iCCP, PNG_INFO_sPLT,
		    PNG_INFO_sCAL, PNG_INFO_IDAT

       For a more compact example of reading a PNG image, see the  file	 exam‐
       ple.c.

   Reading PNG files progressively
       The  progressive	 reader is slightly different from the non-progressive
       reader.	 Instead  of  calling  png_read_info(),	 png_read_rows(),  and
       png_read_end(),	you  make  one call to png_process_data(), which calls
       callbacks when it has the info, a row, or the end of  the  image.   You
       set  up	these callbacks with png_set_progressive_read_fn().  You don't
       have to worry about the input/output functions of libpng,  as  you  are
       giving  the  library  the  data directly in png_process_data().	I will
       assume that you have read the section on reading PNG files above, so  I
       will  only  highlight  the differences (although I will show all of the
       code).

       png_structp png_ptr; png_infop info_ptr;

	/*  An example code fragment of how you would
	    initialize the progressive reader in your
	    application. */
	int
	initialize_png_reader()
	{
	   png_ptr = png_create_read_struct
	       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
		user_error_fn, user_warning_fn);

	   if (!png_ptr)
	       return (ERROR);

	   info_ptr = png_create_info_struct(png_ptr);

	   if (!info_ptr)
	   {
	      png_destroy_read_struct(&png_ptr,
		 (png_infopp)NULL, (png_infopp)NULL);
	      return (ERROR);
	   }

	   if (setjmp(png_jmpbuf(png_ptr)))
	   {
	      png_destroy_read_struct(&png_ptr, &info_ptr,
		 (png_infopp)NULL);
	      return (ERROR);
	   }

	   /* This one's new.  You can provide functions
	      to be called when the header info is valid,
	      when each row is completed, and when the image
	      is finished.  If you aren't using all functions,
	      you can specify NULL parameters.	Even when all
	      three functions are NULL, you need to call
	      png_set_progressive_read_fn().  You can use
	      any struct as the user_ptr (cast to a void pointer
	      for the function call), and retrieve the pointer
	      from inside the callbacks using the function

		 png_get_progressive_ptr(png_ptr);

	      which will return a void pointer, which you have
	      to cast appropriately.
	    */
	   png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
	       info_callback, row_callback, end_callback);

	   return 0;
	}

	/* A code fragment that you call as you receive blocks
	  of data */
	int
	process_data(png_bytep buffer, png_uint_32 length)
	{
	   if (setjmp(png_jmpbuf(png_ptr)))
	   {
	      png_destroy_read_struct(&png_ptr, &info_ptr,
		  (png_infopp)NULL);
	      return (ERROR);
	   }

	   /* This one's new also.  Simply give it a chunk
	      of data from the file stream (in order, of
	      course).	On machines with segmented memory
	      models machines, don't give it any more than
	      64K.  The library seems to run fine with sizes
	      of 4K. Although you can give it much less if
	      necessary (I assume you can give it chunks of
	      1 byte, I haven't tried less then 256 bytes
	      yet).  When this function returns, you may
	      want to display any rows that were generated
	      in the row callback if you don't already do
	      so there.
	    */
	   png_process_data(png_ptr, info_ptr, buffer, length);

	   /* At this point you can call png_process_data_skip if
	      you want to handle data the library will skip yourself;
	      it simply returns the number of bytes to skip (and stops
	      libpng skipping that number of bytes on the next
	      png_process_data call).
	   return 0;
	}

	/* This function is called (as set by
	   png_set_progressive_read_fn() above) when enough data
	   has been supplied so all of the header has been
	   read.
	*/
	void
	info_callback(png_structp png_ptr, png_infop info)
	{
	   /* Do any setup here, including setting any of
	      the transformations mentioned in the Reading
	      PNG files section.  For now, you _must_ call
	      either png_start_read_image() or
	      png_read_update_info() after all the
	      transformations are set (even if you don't set
	      any).  You may start getting rows before
	      png_process_data() returns, so this is your
	      last chance to prepare for that.

	      This is where you turn on interlace handling,
	      assuming you don't want to do it yourself.

	      If you need to you can stop the processing of
	      your original input data at this point by calling
	      png_process_data_pause.  This returns the number
	      of unprocessed bytes from the last png_process_data
	      call - it is up to you to ensure that the next call
	      sees these bytes again.  If you don't want to bother
	      with this you can get libpng to cache the unread
	      bytes by setting the 'save' parameter (see png.h) but
	      then libpng will have to copy the data internally.
	    */
	}

	/* This function is called when each row of image
	   data is complete */
	void
	row_callback(png_structp png_ptr, png_bytep new_row,
	   png_uint_32 row_num, int pass)
	{
	   /* If the image is interlaced, and you turned
	      on the interlace handler, this function will
	      be called for every row in every pass.  Some
	      of these rows will not be changed from the
	      previous pass.  When the row is not changed,
	      the new_row variable will be NULL.  The rows
	      and passes are called in order, so you don't
	      really need the row_num and pass, but I'm
	      supplying them because it may make your life
	      easier.

	      If you did not turn on interlace handling then
	      the callback is called for each row of each
	      sub-image when the image is interlaced.  In this
	      case 'row_num' is the row in the sub-image, not
	      the row in the output image as it is in all other
	      cases.

	      For the non-NULL rows of interlaced images when
	      you have switched on libpng interlace handling,
	      you must call png_progressive_combine_row()
	      passing in the row and the old row.  You can
	      call this function for NULL rows (it will just
	      return) and for non-interlaced images (it just
	      does the memcpy for you) if it will make the
	      code easier.  Thus, you can just do this for
	      all cases if you switch on interlace handling;
	    */

	       png_progressive_combine_row(png_ptr, old_row,
		 new_row);

	   /* where old_row is what was displayed for
	      previously for the row.  Note that the first
	      pass (pass == 0, really) will completely cover
	      the old row, so the rows do not have to be
	      initialized.  After the first pass (and only
	      for interlaced images), you will have to pass
	      the current row, and the function will combine
	      the old row and the new row.

	      You can also call png_process_data_pause in this
	      callback - see above.
	   */
	}

	void
	end_callback(png_structp png_ptr, png_infop info)
	{
	   /* This function is called after the whole image
	      has been read, including any chunks after the
	      image (up to and including the IEND).  You
	      will usually have the same info chunk as you
	      had in the header, although some data may have
	      been added to the comments and time fields.

	      Most people won't do much here, perhaps setting
	      a flag that marks the image as finished.
	    */
	}

IV. Writing
       Much of this is very similar to reading.	 However, everything of impor‐
       tance is repeated here, so you won't have to constantly look back up in
       the reading section to understand writing.

   Setup
       You will want to do the I/O initialization before you get into  libpng,
       so  if it doesn't work, you don't have anything to undo. If you are not
       using the standard I/O functions, you will need to  replace  them  with
       custom writing functions.  See the discussion under Customizing libpng.

	   FILE *fp = fopen(file_name, "wb");

	   if (!fp)
	      return (ERROR);

       Next, png_struct and png_info need to be allocated and initialized.  As
       these can be both relatively large, you may not want to store these  on
       the  stack,  unless you have stack space to spare.  Of course, you will
       want to check if they return NULL.  If you are also reading, you	 won't
       want  to	 name  your  read  structure  and  your	 write	structure both
       "png_ptr"; you can call them anything you like, such as "read_ptr"  and
       "write_ptr".  Look at pngtest.c, for example.

	   png_structp png_ptr = png_create_write_struct
	      (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
	       user_error_fn, user_warning_fn);

	   if (!png_ptr)
	      return (ERROR);

	   png_infop info_ptr = png_create_info_struct(png_ptr);
	   if (!info_ptr)
	   {
	      png_destroy_write_struct(&png_ptr,
		  (png_infopp)NULL);
	      return (ERROR);
	   }

       If  you	want  to  use  your  own  memory  allocation  routines, define
       PNG_USER_MEM_SUPPORTED and use png_create_write_struct_2()  instead  of
       png_create_write_struct():

	   png_structp png_ptr = png_create_write_struct_2
	      (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
	       user_error_fn, user_warning_fn, (png_voidp)
	       user_mem_ptr, user_malloc_fn, user_free_fn);

       After you have these structures, you will need to set up the error han‐
       dling.  When libpng encounters an error, it expects to  longjmp()  back
       to  your	 routine.   Therefore, you will need to call setjmp() and pass
       the png_jmpbuf(png_ptr).	 If you write the  file	 from  different  rou‐
       tines,  you  will need to update the png_jmpbuf(png_ptr) every time you
       enter a new routine that will call a png_*() function.  See your	 docu‐
       mentation  of  setjmp/longjmp for your compiler for more information on
       setjmp/longjmp.	See the discussion on libpng  error  handling  in  the
       Customizing  Libpng  section  below  for more information on the libpng
       error handling.

	   if (setjmp(png_jmpbuf(png_ptr)))
	   {
	   png_destroy_write_struct(&png_ptr, &info_ptr);
	      fclose(fp);
	      return (ERROR);
	   }
	   ...
	   return;

       If you would rather avoid the complexity of setjmp/longjmp issues,  you
       can compile libpng with PNG_NO_SETJMP, in which case errors will result
       in a call to PNG_ABORT() which defaults to abort().

       You can #define PNG_ABORT() to a function that does something more use‐
       ful than abort(), as long as your function does not return.

       Checking for invalid palette index on write was added at libpng 1.5.10.
       If a pixel contains an invalid (out-of-range)  index  libpng  issues  a
       benign  error.  This is enabled by default because this condition is an
       error according to the PNG specification, Clause 11.3.2, but the	 error
       can be ignored in each png_ptr with

	  png_set_check_for_invalid_index(png_ptr, 0);

       If  the error is ignored, or if png_benign_error() treats it as a warn‐
       ing, any invalid pixels are written as-is by the encoder, resulting  in
       an  invalid  PNG datastream as output.  In this case the application is
       responsible for ensuring that the pixel indexes are in  range  when  it
       writes a PLTE chunk with fewer entries than the bit depth would allow.

       Now  you	 need to set up the output code.  The default for libpng is to
       use the C function fwrite().  If you use this, you will need to pass  a
       valid  FILE  * in the function png_init_io().  Be sure that the file is
       opened in binary mode.  Again, if you wish to handle  writing  data  in
       another way, see the discussion on libpng I/O handling in the Customiz‐
       ing Libpng section below.

	   png_init_io(png_ptr, fp);

       If you are embedding your PNG into a datastream such as MNG, and	 don't
       want libpng to write the 8-byte signature, or if you have already writ‐
       ten the signature in your application, use

	   png_set_sig_bytes(png_ptr, 8);

       to inform libpng that it should not write a signature.

   Write callbacks
       At this point, you can set up a callback function that will  be	called
       after  each  row	 has  been  written,  which  you  can use to control a
       progress meter or the like.  It's demonstrated in pngtest.c.  You  must
       supply a function

	   void write_row_callback(png_structp png_ptr, png_uint_32 row,
	      int pass);
	   {
	     /* put your code here */
	   }

       (You can give it another name that you like instead of "write_row_call‐
       back")

       To inform libpng about your function, use

	   png_set_write_status_fn(png_ptr, write_row_callback);

       When this function is called the row has already been  completely  pro‐
       cessed and it has also been written out.	 The 'row' and 'pass' refer to
       the next row to be handled.  For the non-interlaced case the  row  that
       was  just handled is simply one less than the passed in row number, and
       pass will always be 0.  For the interlaced case the same applies unless
       the row value is 0, in which case the row just handled was the last one
       from one of the preceding passes.  Because interlacing may skip a  pass
       you  cannot  be	sure  that the preceding pass is just 'pass-1', if you
       really need to know what the last pass is record	 (row,pass)  from  the
       callback and use the last recorded value each time.

       As  with	 the  user  transform  you  can	 find the output row using the
       PNG_ROW_FROM_PASS_ROW macro.

       You now have the option of modifying how the compression	 library  will
       run.  The following functions are mainly for testing, but may be useful
       in some cases, like if you need to write PNG files extremely  fast  and
       are willing to give up some compression, or if you want to get the max‐
       imum possible compression at the expense of  slower  writing.   If  you
       have no special needs in this area, let the library do what it wants by
       not calling this function at all, as it has been	 tuned	to  deliver  a
       good  speed/compression ratio. The second parameter to png_set_filter()
       is the filter method, for which the only valid values are 0 (as of  the
       July  1999  PNG specification, version 1.2) or 64 (if you are writing a
       PNG datastream that is to be embedded in a MNG datastream).  The	 third
       parameter  is  a	 flag  that  indicates	which filter type(s) are to be
       tested for each scanline.  See the PNG specification for details on the
       specific filter types.

	   /* turn on or off filtering, and/or choose
	      specific filters.	 You can use either a single
	      PNG_FILTER_VALUE_NAME or the bitwise OR of one
	      or more PNG_FILTER_NAME masks.
	    */
	   png_set_filter(png_ptr, 0,
	      PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |
	      PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |
	      PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |
	      PNG_FILTER_AVG   | PNG_FILTER_VALUE_AVG  |
	      PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
	      PNG_ALL_FILTERS);

       If an application wants to start and stop using particular filters dur‐
       ing compression, it should start out with all of the filters (to ensure
       that  the  previous  row	 of  pixels will be stored in case it's needed
       later), and then add and remove them after the start of compression.

       If you are writing a PNG datastream that is to be  embedded  in	a  MNG
       datastream, the second parameter can be either 0 or 64.

       The png_set_compression_*() functions interface to the zlib compression
       library, and should mostly be ignored unless you really know  what  you
       are   doing.   The  only	 generally  useful  call  is  png_set_compres‐
       sion_level() which changes how much time zlib spends on trying to  com‐
       press  the  image  data.	 See the Compression Library (zlib.h and algo‐
       rithm.txt, distributed with zlib) for details on the  compression  lev‐
       els.

	   #include zlib.h

	   /* Set the zlib compression level */
	   png_set_compression_level(png_ptr,
	       Z_BEST_COMPRESSION);

	   /* Set other zlib parameters for compressing IDAT */
	   png_set_compression_mem_level(png_ptr, 8);
	   png_set_compression_strategy(png_ptr,
	       Z_DEFAULT_STRATEGY);
	   png_set_compression_window_bits(png_ptr, 15);
	   png_set_compression_method(png_ptr, 8);
	   png_set_compression_buffer_size(png_ptr, 8192)

	   /* Set zlib parameters for text compression
	    * If you don't call these, the parameters
	    * fall back on those defined for IDAT chunks
	    */
	   png_set_text_compression_mem_level(png_ptr, 8);
	   png_set_text_compression_strategy(png_ptr,
	       Z_DEFAULT_STRATEGY);
	   png_set_text_compression_window_bits(png_ptr, 15);
	   png_set_text_compression_method(png_ptr, 8);

   Setting the contents of info for output
       You  now	 need  to fill in the png_info structure with all the data you
       wish to write before the actual image.  Note that the  only  thing  you
       are  allowed  to	 write after the image is the text chunks and the time
       chunk (as of PNG Specification 1.2, anyway).  See  png_write_end()  and
       the latest PNG specification for more information on that.  If you wish
       to write them before the image, fill them in now, and flag that data as
       being valid.  If you want to wait until after the data, don't fill them
       until png_write_end().  For all the fields in png_info and  their  data
       types, see png.h.  For explanations of what the fields contain, see the
       PNG specification.

       Some of the more important parts of the png_info are:

	   png_set_IHDR(png_ptr, info_ptr, width, height,
	      bit_depth, color_type, interlace_type,
	      compression_type, filter_method)

	   width	  - holds the width of the image
			    in pixels (up to 2^31).

	   height	  - holds the height of the image
			    in pixels (up to 2^31).

	   bit_depth	  - holds the bit depth of one of the
			    image channels.
			    (valid values are 1, 2, 4, 8, 16
			    and depend also on the
			    color_type.	 See also significant
			    bits (sBIT) below).

	   color_type	  - describes which color/alpha
			    channels are present.
			    PNG_COLOR_TYPE_GRAY
			       (bit depths 1, 2, 4, 8, 16)
			    PNG_COLOR_TYPE_GRAY_ALPHA
			       (bit depths 8, 16)
			    PNG_COLOR_TYPE_PALETTE
			       (bit depths 1, 2, 4, 8)
			    PNG_COLOR_TYPE_RGB
			       (bit_depths 8, 16)
			    PNG_COLOR_TYPE_RGB_ALPHA
			       (bit_depths 8, 16)

			    PNG_COLOR_MASK_PALETTE
			    PNG_COLOR_MASK_COLOR
			    PNG_COLOR_MASK_ALPHA

	   interlace_type - PNG_INTERLACE_NONE or
			    PNG_INTERLACE_ADAM7

	   compression_type - (must be
			    PNG_COMPRESSION_TYPE_DEFAULT)

	   filter_method  - (must be PNG_FILTER_TYPE_DEFAULT
			    or, if you are writing a PNG to
			    be embedded in a MNG datastream,
			    can also be
			    PNG_INTRAPIXEL_DIFFERENCING)

       If you call png_set_IHDR(), the call must  appear  before  any  of  the
       other  png_set_*() functions, because they might require access to some
       of the IHDR settings.   The  remaining  png_set_*()  functions  can  be
       called in any order.

       If  you	wish,  you  can reset the compression_type, interlace_type, or
       filter_method later by calling png_set_IHDR() again; if	you  do	 this,
       the  width,  height, bit_depth, and color_type must be the same in each
       call.

	   png_set_PLTE(png_ptr, info_ptr, palette,
	      num_palette);

	   palette	  - the palette for the file
			    (array of png_color)
	   num_palette	  - number of entries in the palette

	   png_set_gAMA(png_ptr, info_ptr, file_gamma);
	   png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);

	   file_gamma	  - the gamma at which the image was
			    created (PNG_INFO_gAMA)

	   int_file_gamma - 100,000 times the gamma at which
			    the image was created

	   png_set_cHRM(png_ptr, info_ptr,  white_x, white_y, red_x, red_y,
			    green_x, green_y, blue_x, blue_y)
	   png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X,
			    green_Y, green_Z, blue_X, blue_Y, blue_Z)
	   png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y,
			    int_red_x, int_red_y, int_green_x, int_green_y,
			    int_blue_x, int_blue_y)
	   png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y,
			    int_red_Z, int_green_X, int_green_Y, int_green_Z,
			    int_blue_X, int_blue_Y, int_blue_Z)

	   {white,red,green,blue}_{x,y}
			    A color space encoding specified using  the	 chro‐
       maticities
			    of the end points and the white point.

	   {red,green,blue}_{X,Y,Z}
			    A  color space encoding specified using the encod‐
       ing end
			    points - the CIE tristimulus specification of  the
       intended
			    color  of  the red, green and blue channels in the
       PNG RGB
			    data.  The white point is simply the  sum  of  the
       three end
			    points.

	   png_set_sRGB(png_ptr, info_ptr, srgb_intent);

	   srgb_intent	  - the rendering intent
			    (PNG_INFO_sRGB) The presence of
			    the sRGB chunk means that the pixel
			    data is in the sRGB color space.
			    This chunk also implies specific
			    values of gAMA and cHRM.  Rendering
			    intent is the CSS-1 property that
			    has been defined by the International
			    Color Consortium
			    (http://www.color.org).
			    It can be one of
			    PNG_sRGB_INTENT_SATURATION,
			    PNG_sRGB_INTENT_PERCEPTUAL,
			    PNG_sRGB_INTENT_ABSOLUTE, or
			    PNG_sRGB_INTENT_RELATIVE.

	   png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
	      srgb_intent);

	   srgb_intent	  - the rendering intent
			    (PNG_INFO_sRGB) The presence of the
			    sRGB chunk means that the pixel
			    data is in the sRGB color space.
			    This function also causes gAMA and
			    cHRM chunks with the specific values
			    that are consistent with sRGB to be
			    written.

	   png_set_iCCP(png_ptr, info_ptr, name, compression_type,
			      profile, proflen);

	   name		    - The profile name.

	   compression_type - The compression type; always
			      PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
			      You may give NULL to this argument to
			      ignore it.

	   profile	    - International Color Consortium color
			      profile data. May contain NULs.

	   proflen	    - length of profile data in bytes.

	   png_set_sBIT(png_ptr, info_ptr, sig_bit);

	   sig_bit	  - the number of significant bits for
			    (PNG_INFO_sBIT) each of the gray, red,
			    green, and blue channels, whichever are
			    appropriate for the given color type
			    (png_color_16)

	   png_set_tRNS(png_ptr, info_ptr, trans_alpha,
	      num_trans, trans_color);

	   trans_alpha	  - array of alpha (transparency)
			    entries for palette (PNG_INFO_tRNS)

	   num_trans	  - number of transparent entries
			    (PNG_INFO_tRNS)

	   trans_color	  - graylevel or color sample values
			    (in order red, green, blue) of the
			    single transparent color for
			    non-paletted images (PNG_INFO_tRNS)

	   png_set_hIST(png_ptr, info_ptr, hist);

	   hist		  - histogram of palette (array of
			    png_uint_16) (PNG_INFO_hIST)

	   png_set_tIME(png_ptr, info_ptr, mod_time);

	   mod_time	  - time image was last modified
			    (PNG_VALID_tIME)

	   png_set_bKGD(png_ptr, info_ptr, background);

	   background	  - background color (of type
			    png_color_16p) (PNG_VALID_bKGD)

	   png_set_text(png_ptr, info_ptr, text_ptr, num_text);

	   text_ptr	  - array of png_text holding image
			    comments

	   text_ptr[i].compression - type of compression used
			on "text" PNG_TEXT_COMPRESSION_NONE
				  PNG_TEXT_COMPRESSION_zTXt
				  PNG_ITXT_COMPRESSION_NONE
				  PNG_ITXT_COMPRESSION_zTXt
	   text_ptr[i].key   - keyword for comment.  Must contain
			1-79 characters.
	   text_ptr[i].text  - text comments for current
				keyword.  Can be NULL or empty.
	   text_ptr[i].text_length - length of text string,
			after decompression, 0 for iTXt
	   text_ptr[i].itxt_length - length of itxt string,
			after decompression, 0 for tEXt/zTXt
	   text_ptr[i].lang  - language of comment (NULL or
				empty for unknown).
	   text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
				or empty for unknown).

	   Note that the itxt_length, lang, and lang_key
	   members of the text_ptr structure only exist when the
	   library is built with iTXt chunk support.  Prior to
	   libpng-1.4.0 the library was built by default without
	   iTXt support. Also note that when iTXt is supported,
	   they contain NULL pointers when the "compression"
	   field contains PNG_TEXT_COMPRESSION_NONE or
	   PNG_TEXT_COMPRESSION_zTXt.

	   num_text	  - number of comments

	   png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
	      num_spalettes);

	   palette_ptr	  - array of png_sPLT_struct structures
			    to be added to the list of palettes
			    in the info structure.
	   num_spalettes  - number of palette structures to be
			    added.

	   png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
	       unit_type);

	   offset_x  - positive offset from the left
			    edge of the screen

	   offset_y  - positive offset from the top
			    edge of the screen

	   unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

	   png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
	       unit_type);

	   res_x       - pixels/unit physical resolution
			 in x direction

	   res_y       - pixels/unit physical resolution
			 in y direction

	   unit_type   - PNG_RESOLUTION_UNKNOWN,
			 PNG_RESOLUTION_METER

	   png_set_sCAL(png_ptr, info_ptr, unit, width, height)

	   unit	       - physical scale units (an integer)

	   width       - width of a pixel in physical scale units

	   height      - height of a pixel in physical scale units
			 (width and height are doubles)

	   png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)

	   unit	       - physical scale units (an integer)

	   width       - width of a pixel in physical scale units
			 expressed as a string

	   height      - height of a pixel in physical scale units
			(width and height are strings like "2.54")

	   png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
	      num_unknowns)

	   unknowns	     - array of png_unknown_chunk
			       structures holding unknown chunks
	   unknowns[i].name  - name of unknown chunk
	   unknowns[i].data  - data of unknown chunk
	   unknowns[i].size  - size of unknown chunk's data
	   unknowns[i].location - position to write chunk in file
				  0: do not write chunk
				  PNG_HAVE_IHDR: before PLTE
				  PNG_HAVE_PLTE: before IDAT
				  PNG_AFTER_IDAT: after IDAT

       The  "location"	member	is set automatically according to what part of
       the output file has already been written.  You  can  change  its	 value
       after  calling  png_set_unknown_chunks()	 as demonstrated in pngtest.c.
       Within each of the "locations", the chunks are sequenced	 according  to
       their  position	in  the structure (that is, the value of "i", which is
       the order in which the chunk was either read from  the  input  file  or
       defined with png_set_unknown_chunks).

       A  quick	 word  about  text and num_text.  text is an array of png_text
       structures.  num_text is the number of valid structures in  the	array.
       Each png_text structure holds a language code, a keyword, a text value,
       and a compression type.

       The compression types have the same valid numbers  as  the  compression
       types  of  the  image  data.  Currently, the only valid number is zero.
       However, you can store text either compressed or	 uncompressed,	unlike
       images,	which  always have to be compressed.  So if you don't want the
       text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
       Because	tEXt and zTXt chunks don't have a language field, if you spec‐
       ify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt any language
       code or translated keyword will not be written out.

       Until text gets around a few hundred bytes, it is not worth compressing
       it.  After the text has been written out to the file,  the  compression
       type   is  set  to  PNG_TEXT_COMPRESSION_NONE_WR	 or  PNG_TEXT_COMPRES‐
       SION_zTXt_WR, so that it isn't written out again at the	end  (in  case
       you are calling png_write_end() with the same struct).

       The keywords that are given in the PNG Specification are:

	   Title	    Short (one line) title or
			    caption for image

	   Author	    Name of image's creator

	   Description	    Description of image (possibly long)

	   Copyright	    Copyright notice

	   Creation Time    Time of original image creation
			    (usually RFC 1123 format, see below)

	   Software	    Software used to create the image

	   Disclaimer	    Legal disclaimer

	   Warning	    Warning of nature of content

	   Source	    Device used to create the image

	   Comment	    Miscellaneous comment; conversion
			    from other image format

       The keyword-text pairs work like this.  Keywords should be short simple
       descriptions of what the comment is about.  Some typical	 keywords  are
       found in the PNG specification, as is some recommendations on keywords.
       You can repeat keywords in a file.  You can even write some text before
       the  image and some after.  For example, you may want to put a descrip‐
       tion of the image before the image,  but	 leave	the  disclaimer	 until
       after, so viewers working over modem connections don't have to wait for
       the disclaimer to go over the modem before they start seeing the image.
       Finally,	 keywords  should  be full words, not abbreviations.  Keywords
       and text are in the ISO 8859-1 (Latin-1) character set (a  superset  of
       regular	ASCII) and can not contain NUL characters, and should not con‐
       tain control or other unprintable characters.   To  make	 the  comments
       widely  readable,  stick	 with  basic ASCII, and avoid machine specific
       character set extensions like the IBM-PC character  set.	  The  keyword
       must  be	 present,  but	you  can leave off the text string on non-com‐
       pressed pairs.  Compressed pairs must have a text string, as  only  the
       text  string is compressed anyway, so the compression would be meaning‐
       less.

       PNG supports modification time via the png_time structure.  Two conver‐
       sion  routines  are  provided, png_convert_from_time_t() for time_t and
       png_convert_from_struct_tm() for struct tm.  The	 time_t	 routine  uses
       gmtime().   You	don't  have to use either of these, but if you wish to
       fill in the png_time structure directly, you should provide the time in
       universal time (GMT) if possible instead of your local time.  Note that
       the year number is the full year (e.g. 1998, rather than 98  -  PNG  is
       year 2000 compliant!), and that months start with 1.

       If  you	want  to  store	 the  time of the original image creation, you
       should use a plain tEXt chunk with the "Creation Time"  keyword.	  This
       is  necessary  because  the  "creation time" of a PNG image is somewhat
       vague, depending on whether you mean the PNG file, the time  the	 image
       was created in a non-PNG format, a still photo from which the image was
       scanned, or possibly the subject matter itself.	In order to facilitate
       machine-readable dates, it is recommended that the "Creation Time" tEXt
       chunk use RFC 1123 format dates (e.g.  "22  May	1997  18:07:10	GMT"),
       although	 this  isn't  a requirement.  Unlike the tIME chunk, the "Cre‐
       ation Time" tEXt chunk is not expected to be automatically  changed  by
       the  software.	To  facilitate	the  use of RFC 1123 dates, a function
       png_convert_to_rfc1123_buffer(png_ptr, buffer, png_timep)  is  provided
       to convert from PNG time to an RFC 1123 format string.  The caller must
       provide a writeable buffer of at least 29 bytes.

   Writing unknown chunks
       You can use the png_set_unknown_chunks function	to  queue  up  private
       chunks  for writing.  You give it a chunk name, location, raw data, and
       a size.	You also must use png_set_keep_unknown_chunks() to ensure that
       libpng  will  handle them.  That's all there is to it.  The chunks will
       be  written   by	  the	next   following   png_write_info_before_PLTE,
       png_write_info, or png_write_end function, depending upon the specified
       location.   Any	chunks	previously  read  into	the  info  structure's
       unknown-chunk  list  will also be written out in a sequence that satis‐
       fies the PNG specification's ordering rules.

       Here is an example of writing two private chunks, prVt and miNE:

	   #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
	   /* Set unknown chunk data */
	   png_unknown_chunk unk_chunk[2];
	   strcpy((char *) unk_chunk[0].name, "prVt";
	   unk_chunk[0].data = (unsigned char *) "PRIVATE DATA";
	   unk_chunk[0].size = strlen(unk_chunk[0].data)+1;
	   unk_chunk[0].location = PNG_HAVE_IHDR;
	   strcpy((char *) unk_chunk[1].name, "miNE";
	   unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA";
	   unk_chunk[1].size = strlen(unk_chunk[0].data)+1;
	   unk_chunk[1].location = PNG_AFTER_IDAT;
	   png_set_unknown_chunks(write_ptr, write_info_ptr,
	       unk_chunk, 2);
	   /* Needed because miNE is not safe-to-copy */
	   png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS,
	      (png_bytep) "miNE", 1);
	   # if PNG_LIBPNG_VER < 10600
	     /* Deal with unknown chunk location bug in 1.5.x and earlier */
	     png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR);
	     png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT);
	   # endif
	   # if PNG_LIBPNG_VER < 10500
	     /* PNG_AFTER_IDAT	writes	two  copies  of	 the  chunk  prior  to
       libpng-1.5.0,
	      *	 one before IDAT and another after IDAT, so don't use it; only
       use
	      * PNG_HAVE_IHDR location.	 This call resets the location	previ‐
       ously
	      *	 set  by  assignment  and png_set_unknown_chunk_location() for
       chunk 1.
	      */
	     png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR);
	   # endif
	   #endif

   The high-level write interface
       At this point there are two ways to  proceed;  through  the  high-level
       write  interface,  or through a sequence of low-level write operations.
       You can use the high-level interface if your image data is  present  in
       the  info structure.  All defined output transformations are permitted,
       enabled by the following masks.

	   PNG_TRANSFORM_IDENTITY      No transformation
	   PNG_TRANSFORM_PACKING       Pack 1, 2 and 4-bit samples
	   PNG_TRANSFORM_PACKSWAP      Change order of packed
				       pixels to LSB first
	   PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
	   PNG_TRANSFORM_SHIFT	       Normalize pixels to the
				       sBIT depth
	   PNG_TRANSFORM_BGR	       Flip RGB to BGR, RGBA
				       to BGRA
	   PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
				       to AG
	   PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
				       to transparency
	   PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
	   PNG_TRANSFORM_STRIP_FILLER	     Strip out filler
					     bytes (deprecated).
	   PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
					     filler bytes
	   PNG_TRANSFORM_STRIP_FILLER_AFTER  Strip out trailing
					     filler bytes

       If you have valid image	data  in  the  info  structure	(you  can  use
       png_set_rows()  to  put	image  data  in the info structure), simply do
       this:

	   png_write_png(png_ptr, info_ptr, png_transforms, NULL)

       where png_transforms is an integer containing the bitwise  OR  of  some
       set   of	  transformation   flags.    This   call   is	equivalent  to
       png_write_info(), followed the set of transformations indicated by  the
       transform mask, then png_write_image(), and finally png_write_end().

       (The  final  parameter  of this call is not yet used.  Someday it might
       point to transformation	parameters  required  by  some	future	output
       transform.)

       You  must use png_transforms and not call any png_set_transform() func‐
       tions when you use png_write_png().

   The low-level write interface
       If you are going the low-level route instead,  you  are	now  ready  to
       write  all  the	file  information up to the actual image data.	You do
       this with a call to png_write_info().

	   png_write_info(png_ptr, info_ptr);

       Note that there is  one	transformation	you  may  need	to  do	before
       png_write_info().   In  PNG files, the alpha channel in an image is the
       level of opacity.  If your data is supplied as a level of transparency,
       you  can	 invert	 the  alpha  channel before you write it, so that 0 is
       fully transparent and 255 (in 8-bit or paletted images)	or  65535  (in
       16-bit images) is fully opaque, with

	   png_set_invert_alpha(png_ptr);

       This  must  appear  before  png_write_info()  instead of later with the
       other transformations because in the case of paletted images  the  tRNS
       chunk  data  has	 to  be inverted before the tRNS chunk is written.  If
       your image is not a paletted image, the tRNS data (which in such	 cases
       represents  a single color to be rendered as transparent) won't need to
       be changed, and you  can	 safely	 do  this  transformation  after  your
       png_write_info() call.

       If you need to write a private chunk that you want to appear before the
       PLTE chunk when PLTE is present, you can write  the  PNG	 info  in  two
       steps, and insert code to write your own chunk between them:

	   png_write_info_before_PLTE(png_ptr, info_ptr);
	   png_set_unknown_chunks(png_ptr, info_ptr, ...);
	   png_write_info(png_ptr, info_ptr);

       After  you've  written the file information, you can set up the library
       to handle any special transformations of the image data.	  The  various
       ways  to	 transform  the	 data will be described in the order that they
       should occur.  This is important, as some of  these  change  the	 color
       type and/or bit depth of the data, and some others only work on certain
       color types and bit depths.  Even though each transformation checks  to
       see  if it has data that it can do something with, you should make sure
       to only enable a transformation if it will be valid for the data.   For
       example, don't swap red and blue on grayscale data.

       PNG  files  store RGB pixels packed into 3 or 6 bytes.  This code tells
       the library to strip input data that has 4 or 8 bytes per pixel down to
       3  or  6	 bytes	(or  strip 2 or 4-byte grayscale+filler data to 1 or 2
       bytes per pixel).

	   png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

       where the 0 is unused, and the location is either PNG_FILLER_BEFORE  or
       PNG_FILLER_AFTER,  depending  upon whether the filler byte in the pixel
       is stored XRGB or RGBX.

       PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small  as
       they can, resulting in, for example, 8 pixels per byte for 1 bit files.
       If the data is supplied at 1 pixel per byte, use this code, which  will
       correctly pack the pixels into a single byte:

	   png_set_packing(png_ptr);

       PNG  files  reduce  possible bit depths to 1, 2, 4, 8, and 16.  If your
       data is of another bit depth, you can write an sBIT chunk into the file
       so that decoders can recover the original data if desired.

	   /* Set the true bit depth of the image data */
	   if (color_type & PNG_COLOR_MASK_COLOR)
	   {
	      sig_bit.red = true_bit_depth;
	      sig_bit.green = true_bit_depth;
	      sig_bit.blue = true_bit_depth;
	   }

	   else
	   {
	      sig_bit.gray = true_bit_depth;
	   }

	   if (color_type & PNG_COLOR_MASK_ALPHA)
	   {
	      sig_bit.alpha = true_bit_depth;
	   }

	   png_set_sBIT(png_ptr, info_ptr, &sig_bit);

       If  the	data is stored in the row buffer in a bit depth other than one
       supported by PNG (e.g. 3 bit data in the range 0-7 for  a  4-bit	 PNG),
       this  will scale the values to appear to be the correct bit depth as is
       required by PNG.

	   png_set_shift(png_ptr, &sig_bit);

       PNG files store 16-bit pixels in network byte  order  (big-endian,  ie.
       most significant bits first).  This code would be used if they are sup‐
       plied the other way (little-endian, i.e. least significant bits	first,
       the way PCs store them):

	   if (bit_depth > 8)
	      png_set_swap(png_ptr);

       If  you	are using packed-pixel images (1, 2, or 4 bits/pixel), and you
       need to change the order the pixels are packed into bytes, you can use:

	   if (bit_depth < 8)
	      png_set_packswap(png_ptr);

       PNG files store 3 color pixels in red, green, blue  order.   This  code
       would be used if they are supplied as blue, green, red:

	   png_set_bgr(png_ptr);

       PNG  files describe monochrome as black being zero and white being one.
       This code would be used if the pixels are supplied with	this  reversed
       (black being one and white being zero):

	   png_set_invert_mono(png_ptr);

       Finally,	 you can write your own transformation function if none of the
       existing ones meets your needs.	This is done  by  setting  a  callback
       with

	   png_set_write_user_transform_fn(png_ptr,
	      write_transform_fn);

       You must supply the function

	   void write_transform_fn(png_structp png_ptr, png_row_infop
	      row_info, png_bytep data)

       See  pngtest.c  for  a  working	example.  Your function will be called
       before any of the other transformations are  processed.	 If  supported
       libpng  also  supplies  an  information routine that may be called from
       your callback:

	  png_get_current_row_number(png_ptr);
	  png_get_current_pass_number(png_ptr);

       This returns the current row passed to the transform.  With  interlaced
       images the value returned is the row in the input sub-image image.  Use
       PNG_ROW_FROM_PASS_ROW(row, pass) and  PNG_COL_FROM_PASS_COL(col,	 pass)
       to  find	 the  output  pixel  (x,y) given an interlaced sub-image pixel
       (row,col,pass).

       The discussion of interlace handling above contains more information on
       how to use these values.

       You can also set up a pointer to a user structure for use by your call‐
       back function.

	   png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

       The user_channels  and  user_depth  parameters  of  this	 function  are
       ignored when writing; you can set them to zero as shown.

       You  can	 retrieve  the	pointer	 via  the function png_get_user_trans‐
       form_ptr().  For example:

	   voidp write_user_transform_ptr =
	      png_get_user_transform_ptr(png_ptr);

       It is possible to have libpng flush any pending	output,	 either	 manu‐
       ally,  or automatically after a certain number of lines have been writ‐
       ten.  To flush the output stream a single time call:

	   png_write_flush(png_ptr);

       and to have libpng flush the output stream periodically after a certain
       number of scanlines have been written, call:

	   png_set_flush(png_ptr, nrows);

       Note   that   the   distance   between  rows  is	 from  the  last  time
       png_write_flush() was called, or the first row of the image if  it  has
       never  been  called.   So if you write 50 lines, and then png_set_flush
       25, it will flush the output on the next scanline, and every  25	 lines
       thereafter,  unless  png_write_flush()  is  called before 25 more lines
       have been written.  If nrows is too small (less than about 10 lines for
       a  640 pixel wide RGB image) the image compression may decrease notice‐
       ably (although this may	be  acceptable	for  real-time	applications).
       Infrequent  flushing will only degrade the compression performance by a
       few percent over images that do not use flushing.

   Writing the image data
       That's it for the transformations.  Now you can write the  image	 data.
       The  simplest  way to do this is in one function call.  If you have the
       whole image in memory, you can just call png_write_image()  and	libpng
       will write the image.  You will need to pass in an array of pointers to
       each row.  This function	 automatically	handles	 interlacing,  so  you
       don't  need  to call png_set_interlace_handling() or call this function
       multiple	 times,	 or  any  of   that   other   stuff   necessary	  with
       png_write_rows().

	   png_write_image(png_ptr, row_pointers);

       where row_pointers is:

	   png_byte *row_pointers[height];

       You can point to void or char or whatever you use for pixels.

       If  you	don't  want  to	 write	the  whole  image at once, you can use
       png_write_rows() instead.  If the file is not interlaced, this is  sim‐
       ple:

	   png_write_rows(png_ptr, row_pointers,
	      number_of_rows);

       row_pointers is the same as in the png_write_image() call.

       If  you are just writing one row at a time, you can do this with a sin‐
       gle row_pointer instead of an array of row_pointers:

	   png_bytep row_pointer = row;

	   png_write_row(png_ptr, row_pointer);

       When the file is interlaced, things can get a good  deal	 more  compli‐
       cated.	The  only  currently (as of the PNG Specification version 1.2,
       dated July 1999) defined	 interlacing  scheme  for  PNG	files  is  the
       "Adam7"	interlace scheme, that breaks down an image into seven smaller
       images of varying size.	libpng will build these images for you, or you
       can  do them yourself.  If you want to build them yourself, see the PNG
       specification for details of which pixels to write when.

       If you don't want libpng to handle the interlacing  details,  just  use
       png_set_interlace_handling() and call png_write_rows() the correct num‐
       ber of times to write all the sub-images	 (png_set_interlace_handling()
       returns the number of sub-images.)

       If  you want libpng to build the sub-images, call this before you start
       writing any rows:

	   number_of_passes = png_set_interlace_handling(png_ptr);

       This will return the number  of	passes	needed.	  Currently,  this  is
       seven, but may change if another interlace type is added.

       Then write the complete image number_of_passes times.

	   png_write_rows(png_ptr, row_pointers, number_of_rows);

       Think  carefully	 before you write an interlaced image.	Typically code
       that reads such images reads all the image  data	 into  memory,	uncom‐
       pressed,	 before	 doing	any processing.	 Only code that can display an
       image on the fly can take advantage of the interlacing  and  even  then
       the  image  has	to  be exactly the correct size for the output device,
       because scaling an image requires adjacent pixels  and  these  are  not
       available until all the passes have been read.

       If you do write an interlaced image you will hardly ever need to handle
       the interlacing yourself.  Call	png_set_interlace_handling()  and  use
       the approach described above.

       The  only  time it is conceivable that you will really need to write an
       interlaced image pass-by-pass is when you have read one	pass  by  pass
       and  made some pixel-by-pixel transformation to it, as described in the
       read code above.	 In this case use the PNG_PASS_ROWS and	 PNG_PASS_COLS
       macros to determine the size of each sub-image in turn and simply write
       the rows you obtained from the read code.

   Finishing a sequential write
       After you are finished writing the image, you should finish writing the
       file.   If  you	are interested in writing comments or time, you should
       pass an appropriately filled png_info pointer.  If you are  not	inter‐
       ested, you can pass NULL.

	   png_write_end(png_ptr, info_ptr);

       When you are done, you can free all memory used by libpng like this:

	   png_destroy_write_struct(&png_ptr, &info_ptr);

       It  is  also  possible  to  individually free the info_ptr members that
       point to libpng-allocated storage with the following function:

	   png_free_data(png_ptr, info_ptr, mask, seq)

	   mask	 - identifies data to be freed, a mask
		   containing the bitwise OR of one or
		   more of
		     PNG_FREE_PLTE, PNG_FREE_TRNS,
		     PNG_FREE_HIST, PNG_FREE_ICCP,
		     PNG_FREE_PCAL, PNG_FREE_ROWS,
		     PNG_FREE_SCAL, PNG_FREE_SPLT,
		     PNG_FREE_TEXT, PNG_FREE_UNKN,
		   or simply PNG_FREE_ALL

	   seq	 - sequence number of item to be freed
		   (-1 for all items)

       This function may be  safely  called  when  the	relevant  storage  has
       already	been freed, or has not yet been allocated, or was allocated by
       the user	 and not by libpng,  and will in those cases do nothing.   The
       "seq"  parameter is ignored if only one item of the selected data type,
       such as PLTE, is allowed.  If "seq" is not -1, and multiple  items  are
       allowed for the data type identified in the mask, such as text or sPLT,
       only the n'th item in the structure is freed, where n is "seq".

       If you allocated data such as a palette that you passed	in  to	libpng
       with  png_set_*,	 you  must  not	 free it until just before the call to
       png_destroy_write_struct().

       The default behavior is only to free data that was allocated internally
       by libpng.  This can be changed, so that libpng will not free the data,
       or so that it will free data  that  was	allocated  by  the  user  with
       png_malloc()  or png_calloc() and passed in via a png_set_*() function,
       with

	   png_data_freer(png_ptr, info_ptr, freer, mask)

	   freer  - one of
		      PNG_DESTROY_WILL_FREE_DATA
		      PNG_SET_WILL_FREE_DATA
		      PNG_USER_WILL_FREE_DATA

	   mask	  - which data elements are affected
		    same choices as in png_free_data()

       For example, to transfer responsibility	for  some  data	 from  a  read
       structure to a write structure, you could use

	   png_data_freer(read_ptr, read_info_ptr,
	      PNG_USER_WILL_FREE_DATA,
	      PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

	   png_data_freer(write_ptr, write_info_ptr,
	      PNG_DESTROY_WILL_FREE_DATA,
	      PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

       thereby	briefly reassigning responsibility for freeing to the user but
       immediately afterwards reassigning it once more	to  the	 write_destroy
       function.   Having done this, it would then be safe to destroy the read
       structure and continue to use the PLTE, tRNS,  and  hIST	 data  in  the
       write structure.

       This  function  only affects data that has already been allocated.  You
       can call this function before calling after the	png_set_*()  functions
       to  control whether the user or png_destroy_*() is supposed to free the
       data.  When the user assumes responsibility for libpng-allocated	 data,
       the  application	 must  use  png_free()	to  free it, and when the user
       transfers responsibility to libpng for data that	 the  user  has	 allo‐
       cated, the user must have used png_malloc() or png_calloc() to allocate
       it.

       If you  allocated  text_ptr.text,  text_ptr.lang,  and  text_ptr.trans‐
       lated_keyword  separately,  do  not transfer responsibility for freeing
       text_ptr to libpng, because when libpng fills a png_text	 structure  it
       combines	 these	members	 with the key member, and png_free_data() will
       free only text_ptr.key.	Similarly, if you transfer responsibility  for
       free'ing	 text_ptr  from	 libpng	 to your application, your application
       must not separately free those members.	For a more compact example  of
       writing a PNG image, see the file example.c.

V. Simplified API
       The  simplified	API, which became available in libpng-1.6.0, hides the
       details of both libpng and the PNG file format itself.  It  allows  PNG
       files to be read into a very limited number of in-memory bitmap formats
       or to be written from the same formats.	If these formats do not accom‐
       modate  your needs then you can, and should, use the more sophisticated
       APIs above - these support a wide variety of in-memory  formats	and  a
       wide  variety of sophisticated transformations to those formats as well
       as a wide variety of APIs to manipulate ancilliary information.

       To read a PNG file using the simplified API:

	 1) Declare a 'png_image' structure (see below) on the
	    stack and memset() it to all zero.

	 2) Call the appropriate png_image_begin_read... function.

	 3) Set the png_image 'format' member to the required
	    format and allocate a buffer for the image.

	 4) Call png_image_finish_read to read the image into
	    your buffer.

       There are no restrictions on the format of the PNG  input  itself;  all
       valid  color  types,  bit depths, and interlace methods are acceptable,
       and the input image is transformed as necessary to  the	requested  in-
       memory format during the png_image_finish_read() step.

       To write a PNG file using the simplified API:

	 1) Declare a 'png_image' structure on the stack and memset()
	    it to all zero.

	 2) Initialize the members of the structure that describe the
	    image, setting the 'format' member to the format of the
	    image in memory.

	 3) Call the appropriate png_image_write... function with a
	    pointer to the image to write the PNG data.

       png_image  is  a	 structure  that  describes the in-memory format of an
       image when it is being read or define the in-memory format of an	 image
       that you need to write.	The "png_image" structure contains the follow‐
       ing members:

	  png_uint_32  version Set to PNG_IMAGE_VERSION
	  png_uint_32  width   Image width in pixels (columns)
	  png_uint_32  height  Image height in pixels (rows)
	  png_uint_32  format  Image format as defined below
	  png_uint_32  flags   A bit mask containing informational flags
	  png_controlp opaque  Initialize to NULL, free with png_image_free
	  png_uint_32  colormap_entries; Number of entries in the color-map
	  png_uint_32  warning_or_error;
	  char	       message[64];

       In the  event  of  an  error  or	 warning  the  following  field	 warn‐
       ing_or_error  field  will  be set to a non-zero value and the 'message'
       field will contain a ' ' terminated string with	the  libpng  error  or
       warning	message.  If both warnings and an error were encountered, only
       the error is recorded.  If there are multiple warnings, only the	 first
       one is recorded.

       The  upper 30 bits of this value are reserved; the low two bits contain
       a two bit code such that a value more than 1 indicates a failure in the
       API just called:

	  0 - no warning or error
	  1 - warning
	  2 - error
	  3 - error preceded by warning

       The  pixels (samples) of the image have one to four channels whose com‐
       ponents have original values in the range 0 to 1.0:

	 1: A single gray or luminance channel (G).
	 2: A gray/luminance channel and an alpha channel (GA).
	 3: Three red, green, blue color channels (RGB).
	 4: Three color channels and an alpha channel (RGBA).

       The channels are encoded in one of two ways:

	 a) As a small integer, value 0..255, contained in a single byte.  For
       the  alpha  channel  the	 original  value is simply value/255.  For the
       color or luminance channels the value is encoded according to the  sRGB
       specification  and matches the 8-bit format expected by typical display
       devices.

       The color/gray channels are not scaled (pre-multiplied)	by  the	 alpha
       channel and are suitable for passing to color management software.

	 b)  As	 a value in the range 0..65535, contained in a 2-byte integer.
       All channels can be converted to the  original  value  by  dividing  by
       65535;  all  channels  are linear.  Color channels use the RGB encoding
       (RGB end-points) of the sRGB specification.  This encoding  is  identi‐
       fied by the PNG_FORMAT_FLAG_LINEAR flag below.

       When  an alpha channel is present it is expected to denote pixel cover‐
       age of the color or luminance channels and is returned as an associated
       alpha  channel:	the color/gray channels are scaled (pre-multiplied) by
       the alpha value.

       When  a	color-mapped  image  is	 used	as   a	 result	  of   calling
       png_image_read_colormap	or  png_image_write_colormap  the channels are
       encoded in the color-map and the descriptions above apply to the color-
       map  entries.   The  image  data	 is  encoded  as small integers, value
       0..255, that index the entries in  the  color-map.   One	 integer  (one
       byte) is stored for each pixel.

       PNG_FORMAT_*

       The  #defines to be used in png_image::format.  Each #define identifies
       a particular layout of channel data  and,  if  present,	alpha  values.
       There are separate defines for each of the two channel encodings.

       A  format  is  built up using single bit flag values.  Not all combina‐
       tions are valid: use the bit flag values below  for  testing  a	format
       returned by the read APIs, but set formats from the derived values.

       When reading or writing color-mapped images the format should be set to
       the    format	of    the    entries	in    the    color-map	  then
       png_image_{read,write}_colormap	called	to read or write the color-map
       and set the format correctly for	 the  image  data.   Do	 not  set  the
       PNG_FORMAT_FLAG_COLORMAP bit directly!

       NOTE: libpng can be built with particular features disabled, if you see
       compiler errors because the definition of one of	 the  following	 flags
       has  been  compiled out it is because libpng does not have the required
       support.	 It is possible, however,  for	the  libpng  configuration  to
       enable  the format on just read or just write; in that case you may see
       an error at run time.  You can guard against this by checking  for  the
       definition of:

	  PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED

	  PNG_FORMAT_FLAG_ALPHA	   0x01 format with an alpha channel
	  PNG_FORMAT_FLAG_COLOR	   0x02 color format: otherwise grayscale
	  PNG_FORMAT_FLAG_LINEAR   0x04 png_uint_16 channels else png_byte
	  PNG_FORMAT_FLAG_COLORMAP 0x08 libpng use only
	  PNG_FORMAT_FLAG_BGR	   0x10 BGR colors, else order is RGB
	  PNG_FORMAT_FLAG_AFIRST   0x20 alpha channel comes first

       Supported  formats  are as follows.  Future versions of libpng may sup‐
       port more formats; for compatibility with older versions	 simply	 check
       if  the	format	macro is defined using #ifdef.	These defines describe
       the in-memory layout of the components of the pixels of the image.

       First the single byte formats:

	  PNG_FORMAT_GRAY 0
	  PNG_FORMAT_GA	  PNG_FORMAT_FLAG_ALPHA
	  PNG_FORMAT_AG	  (PNG_FORMAT_GA|PNG_FORMAT_FLAG_AFIRST)
	  PNG_FORMAT_RGB  PNG_FORMAT_FLAG_COLOR
	  PNG_FORMAT_BGR  (PNG_FORMAT_FLAG_COLOR|PNG_FORMAT_FLAG_BGR)
	  PNG_FORMAT_RGBA (PNG_FORMAT_RGB|PNG_FORMAT_FLAG_ALPHA)
	  PNG_FORMAT_ARGB (PNG_FORMAT_RGBA|PNG_FORMAT_FLAG_AFIRST)
	  PNG_FORMAT_BGRA (PNG_FORMAT_BGR|PNG_FORMAT_FLAG_ALPHA)
	  PNG_FORMAT_ABGR (PNG_FORMAT_BGRA|PNG_FORMAT_FLAG_AFIRST)

       Then the linear 2-byte formats.	When naming these "Y" is used to indi‐
       cate  a luminance (gray) channel.  The component order within the pixel
       is always the same - there is no provision for swapping	the  order  of
       the components in the linear format.

	  PNG_FORMAT_LINEAR_Y PNG_FORMAT_FLAG_LINEAR
	  PNG_FORMAT_LINEAR_Y_ALPHA
	     (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_ALPHA)
	  PNG_FORMAT_LINEAR_RGB
	     (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR)
	  PNG_FORMAT_LINEAR_RGB_ALPHA
	     (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR|
	     PNG_FORMAT_FLAG_ALPHA)

       Color-mapped	  formats	are	 obtained      by      calling
       png_image_{read,write}_colormap,	  as   appropriate    after    setting
       png_image::format to the format of the color-map to be read or written.
       Applications may check the value of PNG_FORMAT_FLAG_COLORMAP to see  if
       they  have called the colormap API.  The format of the color-map may be
       extracted using the following macro.

	  PNG_FORMAT_OF_COLORMAP(fmt) ((fmt) & ~PNG_FORMAT_FLAG_COLORMAP)

       PNG_IMAGE macros

       These are convenience macros to derive  information  from  a  png_image
       structure.   The	 PNG_IMAGE_SAMPLE_ macros return values appropriate to
       the actual image sample values - either the entries in the color-map or
       the  pixels  in	the  image.  The PNG_IMAGE_PIXEL_ macros return corre‐
       sponding values for the pixels and will always return 1 after a call to
       png_image_{read,write}_colormap.	  The remaining macros return informa‐
       tion about the rows in the image and the complete image.

       NOTE: All the macros that take a png_image::format parameter  are  com‐
       pile  time  constants  if  the format parameter is, itself, a constant.
       Therefore these macros can be  used  in	array  declarations  and  case
       labels  where  required.	  Similarly  the macros are also pre-processor
       constants (sizeof is not used) so they can be used in #if tests.

       First the information about the samples.

	 PNG_IMAGE_SAMPLE_CHANNELS(fmt)
	   Returns the total number of channels in a given format: 1..4

	 PNG_IMAGE_SAMPLE_COMPONENT_SIZE(fmt)
	   Returns the size in bytes of a  single  component  of  a  pixel  or
       color-map
	   entry (as appropriate) in the image.

	 PNG_IMAGE_SAMPLE_SIZE(fmt)
	   This	 is  the size of the sample data for one sample.  If the image
       is
	   color-mapped it is the size of one color-map entry (and image  pix‐
       els are
	   one byte in size), otherwise it is the size of one image pixel.

	 PNG_IMAGE_COLORMAP_SIZE(fmt)
	  The  size  of the color-map required by the format; this is the size
       of the
	  color-map buffer passed to the png_image_{read,write}_colormap APIs,
       it is
	  a  fixed  number determined by the format so can easily be allocated
       on the
	  stack if necessary.

       #define	PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt)     (PNG_IMAGE_SAM‐
       PLE_CHANNELS(fmt) * 256)
	  /*  The  maximum  size  of  the  color-map  required	by  the format
       expressed in a
	   * count of components.  This can be used to compile-time allocate a
	   * color-map:
	   *
	   *  png_uint_16  colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(lin‐
       ear_fmt)];
	   *
	   *	    png_byte	    colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPO‐
       NENTS(sRGB_fmt)];
	   *
	   * Alternatively use the PNG_IMAGE_COLORMAP_SIZE macro below to  use
       the
	   * information from one of the png_image_begin_read_ APIs and dynam‐
       ically
	   * allocate the required memory.
	   */

       Corresponding information about the pixels

	 PNG_IMAGE_PIXEL_(test,fmt)

	 PNG_IMAGE_PIXEL_CHANNELS(fmt)
	  The number of separate channels (components) in a pixel; 1 for a
	  color-mapped image.

	 PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)   The size,  in  bytes,  of  each
       component in a pixel; 1 for a color-mapped
	  image.

	 PNG_IMAGE_PIXEL_SIZE(fmt)
	  The size, in bytes, of a complete pixel; 1 for a color-mapped image.

       Information about the whole row, or whole image

	 PNG_IMAGE_ROW_STRIDE(image)
	  Returns the total number of components in a single row of the image;
       this
	  is the minimum 'row stride', the minimum count of components between
       each
	  row.	 For  a color-mapped image this is the minimum number of bytes
       in a
	  row.

	 PNG_IMAGE_BUFFER_SIZE(image, row_stride)
	   Returns the size, in bytes, of an image buffer  given  a  png_image
       and a row
	   stride - the number of components to leave space for in each row.

	 PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01
	   This indicates the the RGB values of the in-memory bitmap do not
	   correspond to the red, green and blue end-points defined by sRGB.

	 PNG_IMAGE_FLAG_COLORMAP == 0x02
	   The	PNG  is color-mapped.  If this flag is set png_image_read_col‐
       ormap
	   can be used without further loss of image information.   If	it  is
       not set
	   png_image_read_colormap  will  cause	 significant loss if the image
       has any

       READ APIs

	  The png_image passed to the read APIs must have been initialized  by
       setting
	  the  png_controlp  field  'opaque'  to  NULL (or, better, memset the
       whole thing.)

	  int png_image_begin_read_from_file( png_imagep image,
	    const char *file_name)

	    The named file is opened for read and the image header
	    is filled in from the PNG header in the file.

	  int png_image_begin_read_from_stdio (png_imagep image,
	    FILE* file)

	     The PNG header is read from the stdio FILE object.

	  int png_image_begin_read_from_memory(png_imagep image,
	     png_const_voidp memory, png_size_t size)

	     The PNG header is read from the given memory buffer.

	  int png_image_finish_read(png_imagep image,
	     png_colorp background, void *buffer,
	     png_int_32 row_stride, void *colormap));

	     Finish reading the image into the supplied buffer and
	     clean up the png_image structure.

	     row_stride is the step, in png_byte or png_uint_16 units
	     as appropriate, between adjacent rows.  A positive stride
	     indicates that the top-most row is first in the buffer -
	     the normal top-down arrangement.  A negative stride
	     indicates that the bottom-most row is first in the buffer.

	     background need only be supplied if an alpha channel must
	     be removed from a png_byte format and the removal is to be
	     done by compositing on a solid color; otherwise it may be
	     NULL and any composition will be done directly onto the
	     buffer.  The value is an sRGB color to use for the
	     background, for grayscale output the green channel is used.

	     For linear output removing the alpha channel is always done
	     by compositing on black.

	  void png_image_free(png_imagep image)

	     Free any data allocated by libpng in image->opaque,
	     setting the pointer to NULL.  May be called at any time
	     after the structure is initialized.

       When the simplified API needs to convert between sRGB and  linear  col‐
       orspaces, the actual sRGB transfer curve defined in the sRGB specifica‐
       tion (see the article at	 http://en.wikipedia.org/wiki/SRGB)  is	 used,
       not the gamma=1/2.2 approximation used elsewhere in libpng.

       WRITE APIS

       For  write  you	must  initialize a png_image structure to describe the
       image to be written:

	  version: must be set to PNG_IMAGE_VERSION
	  opaque: must be initialized to NULL
	  width: image width in pixels
	  height: image height in rows
	  format: the format of the data you wish to write
	  flags: set to 0 unless one of the defined flags applies; set
	     PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images
	     where the RGB values do not correspond to the colors in sRGB.
	  colormap_entries: set to the number of entries in the	 color-map  (0
       to 256)

	  int png_image_write_to_file, (png_imagep image,
	     const char *file, int convert_to_8bit, const void *buffer,
	     png_int_32 row_stride, const void *colormap));

	     Write the image to the named file.

	  int png_image_write_to_stdio(png_imagep image, FILE *file,
	     int convert_to_8_bit, const void *buffer,
	     png_int_32 row_stride, const void *colormap)

	     Write the image to the given (FILE*).

       With  all  write	 APIs  if  image  is in one of the linear formats with
       (png_uint_16) data then setting convert_to_8_bit will cause the	output
       to  be  a (png_byte) PNG gamma encoded according to the sRGB specifica‐
       tion, otherwise a 16-bit linear encoded PNG file is written.

       With all APIs row_stride is handled as in the read APIs	-  it  is  the
       spacing	from  one row to the next in component sized units (float) and
       if negative indicates a bottom-up row layout in the buffer.

       Note that the write API does not support interlacing, sub-8-bit pixels,
       and indexed (paletted) images.

VI. Modifying/Customizing libpng
       There are two issues here.  The first is changing how libpng does stan‐
       dard things like memory allocation, input/output, and  error  handling.
       The  second  deals with more complicated things like adding new chunks,
       adding new transformations, and generally changing  how	libpng	works.
       Both  of	 those	are  compile-time  issues; that is, they are generally
       determined at the time the code is written, and there is rarely a  need
       to provide the user with a means of changing them.

       Memory allocation, input/output, and error handling

       All  of	the  memory  allocation,  input/output,	 and error handling in
       libpng goes through callbacks that are user-settable.  The default rou‐
       tines  are  in  pngmem.c,  pngrio.c,  pngwio.c, and pngerror.c, respec‐
       tively.	To change these functions, call the appropriate png_set_*_fn()
       function.

       Memory  allocation is done through the functions png_malloc(), png_cal‐
       loc(), and png_free().  The png_malloc() and png_free() functions  cur‐
       rently  just  call  the	standard  C  functions	and png_calloc() calls
       png_malloc() and then clears the newly allocated memory to  zero;  note
       that  png_calloc(png_ptr,  size)	 is not the same as the calloc(number,
       size) function provided by stdlib.h.  There is limited support for cer‐
       tain  systems  with  segmented  memory  architectures  and the types of
       pointers declared by png.h match this; you will have to use appropriate
       pointers	 in your application.  Since it is unlikely that the method of
       handling memory allocation on a platform will change  between  applica‐
       tions, these functions must be modified in the library at compile time.
       If you prefer to use a different method of allocating and freeing data,
       you  can	 use png_create_read_struct_2() or png_create_write_struct_2()
       to register your own functions as  described  above.   These  functions
       also provide a void pointer that can be retrieved via

	   mem_ptr=png_get_mem_ptr(png_ptr);

       Your replacement memory functions must have prototypes as follows:

	   png_voidp malloc_fn(png_structp png_ptr,
	      png_alloc_size_t size);

	   void free_fn(png_structp png_ptr, png_voidp ptr);

       Your malloc_fn() must return NULL in case of failure.  The png_malloc()
       function will normally call png_error() if it receives a NULL from  the
       system memory allocator or from your replacement malloc_fn().

       Your  free_fn()	will  never  be called with a NULL ptr, since libpng's
       png_free() checks for NULL before calling free_fn().

       Input/Output in libpng is  done	through	 png_read()  and  png_write(),
       which  currently	 just call fread() and fwrite().  The FILE * is stored
       in png_struct and is initialized via png_init_io().   If	 you  wish  to
       change  the  method of I/O, the library supplies callbacks that you can
       set through the function png_set_read_fn()  and	png_set_write_fn()  at
       run  time,  instead of calling the png_init_io() function.  These func‐
       tions also provide a void pointer that can be retrieved via  the	 func‐
       tion png_get_io_ptr().  For example:

	   png_set_read_fn(png_structp read_ptr,
	       voidp read_io_ptr, png_rw_ptr read_data_fn)

	   png_set_write_fn(png_structp write_ptr,
	       voidp write_io_ptr, png_rw_ptr write_data_fn,
	       png_flush_ptr output_flush_fn);

	   voidp read_io_ptr = png_get_io_ptr(read_ptr);
	   voidp write_io_ptr = png_get_io_ptr(write_ptr);

       The replacement I/O functions must have prototypes as follows:

	   void user_read_data(png_structp png_ptr,
	       png_bytep data, png_size_t length);

	   void user_write_data(png_structp png_ptr,
	       png_bytep data, png_size_t length);

	   void user_flush_data(png_structp png_ptr);

       The user_read_data() function is responsible for detecting and handling
       end-of-data errors.

       Supplying NULL for the read, write, or flush functions sets  them  back
       to  using  the  default	C stream functions, which expect the io_ptr to
       point to a standard *FILE structure.  It is probably a mistake  to  use
       NULL for one of write_data_fn and output_flush_fn but not both of them,
       unless you have built libpng with PNG_NO_WRITE_FLUSH defined.  It is an
       error to read from a write stream, and vice versa.

       Error handling in libpng is done through png_error() and png_warning().
       Errors handled through png_error() are fatal, meaning that  png_error()
       should  never  return  to  its  caller.	Currently, this is handled via
       setjmp()	 and  longjmp()	 (unless  you  have   compiled	 libpng	  with
       PNG_NO_SETJMP,  in  which  case it is handled via PNG_ABORT()), but you
       could change this to do things like exit() if you should wish, as  long
       as your function does not return.

       On  non-fatal  errors,  png_warning() is called to print a warning mes‐
       sage, and then  control	returns	 to  the  calling  code.   By  default
       png_error()  and	 png_warning() print a message on stderr via fprintf()
       unless the library is compiled with PNG_NO_CONSOLE_IO defined  (because
       you don't want the messages) or PNG_NO_STDIO defined (because fprintf()
       isn't available).  If you wish to change	 the  behavior	of  the	 error
       functions,  you	will need to set up your own message callbacks.	 These
       functions are normally supplied at the time that the png_struct is cre‐
       ated.   It is also possible to redirect errors and warnings to your own
       replacement functions after png_create_*_struct() has  been  called  by
       calling:

	   png_set_error_fn(png_structp png_ptr,
	       png_voidp error_ptr, png_error_ptr error_fn,
	       png_error_ptr warning_fn);

	   png_voidp error_ptr = png_get_error_ptr(png_ptr);

       If  NULL is supplied for either error_fn or warning_fn, then the libpng
       default function will be used, calling fprintf() and/or longjmp() if  a
       problem	is  encountered.   The replacement error functions should have
       parameters as follows:

	   void user_error_fn(png_structp png_ptr,
	       png_const_charp error_msg);

	   void user_warning_fn(png_structp png_ptr,
	       png_const_charp warning_msg);

       The motivation behind using setjmp() and longjmp() is the C++ throw and
       catch  exception	 handling methods.  This makes the code much easier to
       write, as there is no need to check every return code of every function
       call.   However, there are some uncertainties about the status of local
       variables after a longjmp, so the user may want	to  be	careful	 about
       doing  anything after setjmp returns non-zero besides returning itself.
       Consult your compiler documentation for more details.  For an  alterna‐
       tive  approach,	you  may  wish	to  use	 the  "cexcept"	 facility (see
       http://cexcept.sourceforge.net), which is illustrated in pngvalid.c and
       in contrib/visupng.

       Beginning  in  libpng-1.4.0,  the  png_set_benign_errors()  API	became
       available.  You can use this to handle certain errors (normally handled
       as errors) as warnings.

	   png_set_benign_errors (png_ptr, int allowed);

	   allowed: 0: treat png_benign_error() as an error.
		    1: treat png_benign_error() as a warning.

       As  of libpng-1.6.0, the default condition is to treat benign errors as
       warnings while reading and as errors while writing.

   Custom chunks
       If you need to read or write custom chunks, you may need to get	deeper
       into  the  libpng code.	The library now has mechanisms for storing and
       writing chunks of unknown type; you can even declare callbacks for cus‐
       tom  chunks.   However, this may not be good enough if the library code
       itself needs to know about interactions between your chunk and existing
       `intrinsic' chunks.

       If you need to write a new intrinsic chunk, first read the PNG specifi‐
       cation. Acquire a first level of understanding of how  it  works.   Pay
       particular  attention  to  the  sections that describe chunk names, and
       look at how other chunks were designed, so you can do things similarly.
       Second,	check  out  the sections of libpng that read and write chunks.
       Try to find a chunk that is similar to yours and use it as a  template.
       More  details can be found in the comments inside the code.  It is best
       to handle private or unknown chunks in a generic method,	 via  callback
       functions,  instead  of	by  modifying libpng functions. This is illus‐
       trated in pngtest.c, which uses a callback function to handle a private
       "vpAg"  chunk  and  the	new  "sTER"  chunk,  which are both unknown to
       libpng.

       If you wish to write your own transformation for the data, look through
       the  part of the code that does the transformations, and check out some
       of the simpler ones to get an idea of how they work.   Try  to  find  a
       similar	transformation	to the one you want to add and copy off of it.
       More details can be found in the comments inside the code itself.

   Configuring for 16-bit platforms
       You will want to look into zconf.h to tell zlib (and thus libpng)  that
       it  cannot allocate more then 64K at a time.  Even if you can, the mem‐
       ory won't be accessible.	 So limit zlib and libpng to 64K  by  defining
       MAXSEG_64K.

   Configuring for DOS
       For  DOS users who only have access to the lower 640K, you will have to
       limit zlib's memory usage via a	png_set_compression_mem_level()	 call.
       See zlib.h or zconf.h in the zlib library for more information.

   Configuring for Medium Model
       Libpng's	 support for medium model has been tested on most of the popu‐
       lar compilers.  Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
       defined,	 and  FAR  gets defined to far in pngconf.h, and you should be
       all set.	 Everything in the library (except for	zlib's	structure)  is
       expecting  far data.  You must use the typedefs with the p or pp on the
       end for pointers (or at least look at them and be careful).  Make  note
       that  the rows of data are defined as png_bytepp, which is an "unsigned
       char far * far *".

   Configuring for gui/windowing platforms:
       You will need to write new error and warning functions that use the GUI
       interface,  as  described  previously, and set them to be the error and
       warning functions at the time that png_create_*_struct() is called,  in
       order to have them available during the structure initialization.  They
       can be changed later via png_set_error_fn().  On	 some  compilers,  you
       may also have to change the memory allocators (png_malloc, etc.).

   Configuring for compiler xxx:
       All  includes  for libpng are in pngconf.h.  If you need to add, change
       or delete an include, this is the place to do it.   The	includes  that
       are  not	 needed	 outside libpng are placed in pngpriv.h, which is only
       used by the routines inside libpng itself.  The files in libpng	proper
       only include pngpriv.h and png.h, which %14%in turn includes pngconf.h.
       in turn includes pngconf.h and, as of libpng-1.5.0,  pnglibconf.h.   As
       of  libpng-1.5.0,  pngpriv.h  also  includes three other private header
       files, pngstruct.h, pnginfo.h, and pngdebug.h, which  contain  material
       that previously appeared in the public headers.

   Configuring zlib:
       There  are special functions to configure the compression.  Perhaps the
       most useful one changes the compression	level,	which  currently  uses
       input compression values in the range 0 - 9.  The library normally uses
       the default compression level (Z_DEFAULT_COMPRESSION = 6).  Tests  have
       shown  that  for	 a large majority of images, compression values in the
       range 3-6 compress nearly as well as higher  levels,  and  do  so  much
       faster.	 For  online  applications it may be desirable to have maximum
       speed (Z_BEST_SPEED = 1).  With versions of zlib after v0.99,  you  can
       also specify no compression (Z_NO_COMPRESSION = 0), but this would cre‐
       ate files larger than just storing the raw bitmap.  You can specify the
       compression level by calling:

	   #include zlib.h
	   png_set_compression_level(png_ptr, level);

       Another	useful	one is to reduce the memory level used by the library.
       The memory level defaults to 8, but it can be lowered if you are	 short
       on  memory  (running DOS, for example, where you only have 640K).  Note
       that the memory level does have an effect on compression;  among	 other
       things,	lower  levels  will  result in sections of incompressible data
       being emitted in smaller stored blocks, with a  correspondingly	larger
       relative overhead of up to 15% in the worst case.

	   #include zlib.h
	   png_set_compression_mem_level(png_ptr, level);

       The other functions are for configuring zlib.  They are not recommended
       for normal use and may result in writing	 an  invalid  PNG  file.   See
       zlib.h for more information on what these mean.

	   #include zlib.h
	   png_set_compression_strategy(png_ptr,
	       strategy);

	   png_set_compression_window_bits(png_ptr,
	       window_bits);

	   png_set_compression_method(png_ptr, method);

	   png_set_compression_buffer_size(png_ptr, size);

       As  of  libpng  version	1.5.4, additional APIs became available to set
       these separately for non-IDAT compressed chunks such as zTXt, iTXt, and
       iCCP:

	   #include zlib.h
	   #if PNG_LIBPNG_VER >= 10504
	   png_set_text_compression_level(png_ptr, level);

	   png_set_text_compression_mem_level(png_ptr, level);

	   png_set_text_compression_strategy(png_ptr,
	       strategy);

	   png_set_text_compression_window_bits(png_ptr,
	       window_bits);

	   png_set_text_compression_method(png_ptr, method);
	   #endif

   Controlling row filtering
       If you want to control whether libpng uses filtering or not, which fil‐
       ters are used, and how it goes about picking row filters, you can  call
       one of these functions.	The selection and configuration of row filters
       can have a significant impact on the size  and  encoding	 speed	and  a
       somewhat lesser impact on the decoding speed of an image.  Filtering is
       enabled by default for RGB  and	grayscale  images  (with  and  without
       alpha),	but not for paletted images nor for any images with bit depths
       less than 8 bits/pixel.

       The 'method' parameter sets the main filtering method,  which  is  cur‐
       rently  only '0' in the PNG 1.2 specification.  The 'filters' parameter
       sets which filter(s), if any, should be used for each scanline.	Possi‐
       ble  values are PNG_ALL_FILTERS and PNG_NO_FILTERS to turn filtering on
       and off, respectively.

       Individual filter types are PNG_FILTER_NONE,  PNG_FILTER_SUB,  PNG_FIL‐
       TER_UP,	PNG_FILTER_AVG,	 PNG_FILTER_PAETH,  which  can be bitwise ORed
       together with '|' to specify one or more filters to use.	 These filters
       are  described  in more detail in the PNG specification.	 If you intend
       to change the filter type during the course of writing the  image,  you
       should start with flags set for all of the filters you intend to use so
       that libpng can initialize its internal	structures  appropriately  for
       all  of	the  filter  types.   (Note that this means the first row must
       always be adaptively filtered, because libpng currently does not	 allo‐
       cate  the  filter buffers until png_write_row() is called for the first
       time.)

	   filters = PNG_FILTER_NONE | PNG_FILTER_SUB
		     PNG_FILTER_UP | PNG_FILTER_AVG |
		     PNG_FILTER_PAETH | PNG_ALL_FILTERS;

	   png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
	      filters);
		     The second parameter can also be
		     PNG_INTRAPIXEL_DIFFERENCING if you are
		     writing a PNG to be embedded in a MNG
		     datastream.  This parameter must be the
		     same as the value of filter_method used
		     in png_set_IHDR().

       It is also possible to influence how  libpng  chooses  from  among  the
       available  filters.   This  is  done  in	 one  or both of two ways - by
       telling it how important it is to keep the same filter  for  successive
       rows,  and  by  telling it the relative computational costs of the fil‐
       ters.

	   double weights[3] = {1.5, 1.3, 1.1},
	      costs[PNG_FILTER_VALUE_LAST] =
	      {1.0, 1.3, 1.3, 1.5, 1.7};

	   png_set_filter_heuristics(png_ptr,
	      PNG_FILTER_HEURISTIC_WEIGHTED, 3,
	      weights, costs);

       The weights are multiplying factors that indicate to  libpng  that  the
       row  filter  should  be the same for successive rows unless another row
       filter is that many times better than  the  previous  filter.   In  the
       above  example,	if the previous 3 filters were SUB, SUB, NONE, the SUB
       filter could have a "sum of  absolute  differences"  1.5	 x  1.3	 times
       higher  than  other  filters and still be chosen, while the NONE filter
       could have a sum 1.1 times higher than other filters and still be  cho‐
       sen.   Unspecified  weights  are	 taken	to  be	1.0, and the specified
       weights should probably be declining  like  those  above	 in  order  to
       emphasize recent filters over older filters.

       The  filter costs specify for each filter type a relative decoding cost
       to be considered when selecting row filters.  This means	 that  filters
       with  higher costs are less likely to be chosen over filters with lower
       costs, unless their "sum of absolute differences" is that much smaller.
       The  costs do not necessarily reflect the exact computational speeds of
       the various filters, since this would unduly influence the final	 image
       size.

       Note  that  the numbers above were invented purely for this example and
       are given only to help explain the function usage.  Little testing  has
       been done to find optimum values for either the costs or the weights.

   Removing unwanted object code
       There  are a bunch of #define's in pngconf.h that control what parts of
       libpng are compiled.  All the defines end in _SUPPORTED.	  If  you  are
       never  going  to use a capability, you can change the #define to #undef
       before recompiling libpng and save yourself code and data space, or you
       can  turn  off  individual  capabilities	 with  defines that begin with
       PNG_NO_.

       In libpng-1.5.0 and later, the #define's are in pnglibconf.h instead.

       You can also turn all of the transforms and ancillary  chunk  capabili‐
       ties  off  en masse with compiler directives that define PNG_NO_READ[or
       WRITE]_TRANSFORMS, or PNG_NO_READ[or  WRITE]_ANCILLARY_CHUNKS,  or  all
       four, along with directives to turn on any of the capabilities that you
       do want.	 The PNG_NO_READ[or WRITE]_TRANSFORMS directives  disable  the
       extra  transformations  but  still  leave  the library fully capable of
       reading and writing PNG files with all known public chunks. Use of  the
       PNG_NO_READ[or  WRITE]_ANCILLARY_CHUNKS	directive  produces  a library
       that is incapable of reading or writing ancillary chunks.  If  you  are
       not  using  the	progressive  reading capability, you can turn that off
       with PNG_NO_PROGRESSIVE_READ (don't confuse this with  the  INTERLACING
       capability, which you'll still have).

       All the reading and writing specific code are in separate files, so the
       linker should only grab the files it needs.  However, if	 you  want  to
       make  sure, or if you are building a stand alone library, all the read‐
       ing files start with "pngr"  and	 all  the  writing  files  start  with
       "pngw".	 The  files  that  don't match either (like png.c, pngtrans.c,
       etc.)  are used for both reading and writing, and  always  need	to  be
       included.  The progressive reader is in pngpread.c

       If you are creating or distributing a dynamically linked library (a .so
       or DLL file), you should	 not  remove  or  disable  any	parts  of  the
       library, as this will cause applications linked with different versions
       of the library to fail if they call functions  not  available  in  your
       library.	  The  size  of	 the  library  itself  should not be an issue,
       because only those sections that are actually used will be loaded  into
       memory.

   Requesting debug printout
       The  macro definition PNG_DEBUG can be used to request debugging print‐
       out.  Set it to an integer value in the range 0 to 3.   Higher  numbers
       result in increasing amounts of debugging information.  The information
       is printed to the "stderr" file, unless another file name is  specified
       in the PNG_DEBUG_FILE macro definition.

       When PNG_DEBUG > 0, the following functions (macros) become available:

	  png_debug(level, message)
	  png_debug1(level, message, p1)
	  png_debug2(level, message, p1, p2)

       in  which  "level"  is compared to PNG_DEBUG to decide whether to print
       the message, "message" is the formatted string to be  printed,  and  p1
       and  p2	are parameters that are to be embedded in the string according
       to printf-style formatting directives.  For example,

	  png_debug1(2, "foo=%d", foo);

       is expanded to

	  if (PNG_DEBUG > 2)
	     fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);

       When PNG_DEBUG is defined but is zero, the macros aren't	 defined,  but
       you can still use PNG_DEBUG to control your own debugging:

	  #ifdef PNG_DEBUG
	      fprintf(stderr, ...
	  #endif

       When  PNG_DEBUG	= 1, the macros are defined, but only png_debug state‐
       ments having level = 0 will be printed.	There aren't any  such	state‐
       ments  in  this	version of libpng, but if you insert some they will be
       printed.

   Prepending a prefix to exported symbols
       Starting with libpng-1.6.0, you can configure libpng  (when  using  the
       "configure" script) to prefix all exported symbols by means of the con‐
       figuration option "--with-libpng-prefix=FOO_", where FOO_  can  be  any
       string beginning with a letter and containing only uppercase and lower‐
       case letters, digits, and the underscore (i.e., a  C  language  identi‐
       fier).  This creates a set of macros in pnglibconf.h, so this is trans‐
       parent to applications; their function calls  get  transformed  by  the
       macros to use the modified names.

VII. MNG support
       The  MNG	 specification	(available  at	http://www.libpng.org/pub/mng)
       allows certain extensions to PNG for PNG images that  are  embedded  in
       MNG  datastreams.   Libpng  can	support	 some of these extensions.  To
       enable them, use the png_permit_mng_features() function:

	  feature_set = png_permit_mng_features(png_ptr, mask)

	  mask is a png_uint_32 containing the bitwise OR of the
	       features you want to enable.  These include
	       PNG_FLAG_MNG_EMPTY_PLTE
	       PNG_FLAG_MNG_FILTER_64
	       PNG_ALL_MNG_FEATURES

	  feature_set is a png_uint_32 that is the bitwise AND of
	     your mask with the set of MNG features that is
	     supported by the version of libpng that you are using.

       It is an error to use this function when reading or  writing  a	stand‐
       alone  PNG file with the PNG 8-byte signature.  The PNG datastream must
       be wrapped in a MNG datastream.	As a minimum, it  must	have  the  MNG
       8-byte signature and the MHDR and MEND chunks.  Libpng does not provide
       support for these or any other MNG chunks; your application  must  pro‐
       vide  its  own support for them.	 You may wish to consider using libmng
       (available at http://www.libmng.com) instead.

VIII. Changes to Libpng from version 0.88
       It should be noted that versions of libpng later than 0.96 are not dis‐
       tributed	 by  the  original libpng author, Guy Schalnat, nor by Andreas
       Dilger, who had taken over from Guy during 1996 and 1997, and  distrib‐
       uted  versions  0.89  through 0.96, but rather by another member of the
       original PNG Group, Glenn Randers-Pehrson.  Guy and Andreas  are	 still
       alive and well, but they have moved on to other things.

       The    old    libpng   functions	  png_read_init(),   png_write_init(),
       png_info_init(), png_read_destroy(), and png_write_destroy() have  been
       moved  to  PNG_INTERNAL in version 0.95 to discourage their use.	 These
       functions will be removed from libpng version 1.4.0.

       The preferred method of creating and initializing the libpng structures
       is  via	the  png_create_read_struct(),	png_create_write_struct(), and
       png_create_info_struct() because they isolate the size  of  the	struc‐
       tures  from  the	 application,  allow  version error checking, and also
       allow the use of custom error handling routines during the  initializa‐
       tion, which the old functions do not.  The functions png_read_destroy()
       and png_write_destroy() do not actually free  the  memory  that	libpng
       allocated  for  these  structs,	but just reset the data structures, so
       they   can   be	 used	instead	  of   png_destroy_read_struct()   and
       png_destroy_write_struct()  if  you feel there is too much system over‐
       head allocating and freeing the png_struct for each image read.

       Setting	 the   error   callbacks   via	 png_set_message_fn()	before
       png_read_init()	as was suggested in libpng-0.88 is no longer supported
       because this caused applications that do not use custom error functions
       to fail if the png_ptr was not initialized to zero.  It is still possi‐
       ble to set the error callbacks AFTER png_read_init(), or to change them
       with  png_set_error_fn(),  which	 is essentially the same function, but
       with a new name to force compilation errors with applications that  try
       to use the old method.

       Support	for  the  sCAL,	 iCCP,	iTXt,  and  sPLT  chunks  was added at
       libpng-1.0.6; however, iTXt support was not enabled by default.

       Starting with version 1.0.7, you can find  out  which  version  of  the
       library you are using at run-time:

	  png_uint_32 libpng_vn = png_access_version_number();

       The  number libpng_vn is constructed from the major version, minor ver‐
       sion with leading zero, and release number with	leading	 zero,	(e.g.,
       libpng_vn for version 1.0.7 is 10007).

       Note  that  this	 function  does not take a png_ptr, so you can call it
       before you've created one.

       You can also check which version of png.h you used when compiling  your
       application:

	  png_uint_32 application_vn = PNG_LIBPNG_VER;

IX. Changes to Libpng from version 1.0.x to 1.2.x
       Support	for  user memory management was enabled by default.  To accom‐
       plish  this,   the   functions	png_create_read_struct_2(),   png_cre‐
       ate_write_struct_2(),   png_set_mem_fn(),  png_get_mem_ptr(),  png_mal‐
       loc_default(), and png_free_default() were added.

       Support for the iTXt chunk has been enabled by default  as  of  version
       1.2.41.

       Support for certain MNG features was enabled.

       Support	for  numbered error messages was added.	 However, we never got
       around  to  actually  numbering	the  error  messages.	The   function
       png_set_strip_error_numbers()  was  added (Note: the prototype for this
       function was inadvertently removed from png.h in	 PNG_NO_ASSEMBLER_CODE
       builds of libpng-1.2.15.	 It was restored in libpng-1.2.36).

       The  png_malloc_warn() function was added at libpng-1.2.3.  This issues
       a png_warning and returns NULL instead of aborting  when	 it  fails  to
       acquire the requested memory allocation.

       Support	for  setting user limits on image width and height was enabled
       by      default.	      The	functions	png_set_user_limits(),
       png_get_user_width_max(),  and  png_get_user_height_max() were added at
       libpng-1.2.6.

       The png_set_add_alpha() function was added at libpng-1.2.7.

       The   function	png_set_expand_gray_1_2_4_to_8()    was	   added    at
       libpng-1.2.9.   Unlike png_set_gray_1_2_4_to_8(), the new function does
       not expand the tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() func‐
       tion is deprecated.

       A number of macro definitions in support of runtime selection of assem‐
       bler code features (especially Intel MMX code support)  were  added  at
       libpng-1.2.0:

	   PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
	   PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
	   PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
	   PNG_ASM_FLAG_MMX_READ_INTERLACE
	   PNG_ASM_FLAG_MMX_READ_FILTER_SUB
	   PNG_ASM_FLAG_MMX_READ_FILTER_UP
	   PNG_ASM_FLAG_MMX_READ_FILTER_AVG
	   PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
	   PNG_ASM_FLAGS_INITIALIZED
	   PNG_MMX_READ_FLAGS
	   PNG_MMX_FLAGS
	   PNG_MMX_WRITE_FLAGS
	   PNG_MMX_FLAGS

       We  added  the  following  functions in support of runtime selection of
       assembler code features:

	   png_get_mmx_flagmask()
	   png_set_mmx_thresholds()
	   png_get_asm_flags()
	   png_get_mmx_bitdepth_threshold()
	   png_get_mmx_rowbytes_threshold()
	   png_set_asm_flags()

       We replaced all of these functions with simple stubs in	libpng-1.2.20,
       when the Intel assembler code was removed due to a licensing issue.

       These macros are deprecated:

	   PNG_READ_TRANSFORMS_NOT_SUPPORTED
	   PNG_PROGRESSIVE_READ_NOT_SUPPORTED
	   PNG_NO_SEQUENTIAL_READ_SUPPORTED
	   PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
	   PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
	   PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED

       They have been replaced, respectively, by:

	   PNG_NO_READ_TRANSFORMS
	   PNG_NO_PROGRESSIVE_READ
	   PNG_NO_SEQUENTIAL_READ
	   PNG_NO_WRITE_TRANSFORMS
	   PNG_NO_READ_ANCILLARY_CHUNKS
	   PNG_NO_WRITE_ANCILLARY_CHUNKS

       PNG_MAX_UINT was replaced with PNG_UINT_31_MAX.	It has been deprecated
       since libpng-1.0.16 and libpng-1.2.6.

       The function
	   png_check_sig(sig, num) was replaced with
	   !png_sig_cmp(sig, 0, num) It has been deprecated since libpng-0.90.

       The function
	   png_set_gray_1_2_4_to_8() which also	 expands  tRNS	to  alpha  was
       replaced with
	   png_set_expand_gray_1_2_4_to_8() which does not. It has been depre‐
       cated since libpng-1.0.18 and 1.2.9.

X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
       Private libpng prototypes and macro definitions were moved  from	 png.h
       and pngconf.h into a new pngpriv.h header file.

       Functions      png_set_benign_errors(),	   png_benign_error(),	   and
       png_chunk_benign_error() were added.

       Support for setting the maximum amount of memory that  the  application
       will allocate for reading chunks was added, as a security measure.  The
       functions png_set_chunk_cache_max() and png_get_chunk_cache_max()  were
       added to the library.

       We implemented support for I/O states by adding png_ptr member io_state
       and  functions  png_get_io_chunk_name()	 and   png_get_io_state()   in
       pngget.c

       We  added  PNG_TRANSFORM_GRAY_TO_RGB  to the available high-level input
       transforms.

       Checking for and reporting of errors in the IHDR chunk  is  more	 thor‐
       ough.

       Support for global arrays was removed, to improve thread safety.

       Some obsolete/deprecated macros and functions have been removed.

       Typecasted NULL definitions such as
	  #define  png_voidp_NULL	      (png_voidp)NULL were eliminated.
       If you used these in your application, just use NULL instead.

       The png_struct and info_struct members "trans" and "trans_values"  were
       changed to "trans_alpha" and "trans_color", respectively.

       The  obsolete,  unused pnggccrd.c and pngvcrd.c files and related make‐
       files were removed.

       The PNG_1_0_X and PNG_1_2_X macros were eliminated.

       The PNG_LEGACY_SUPPORTED macro was eliminated.

       Many WIN32_WCE #ifdefs were removed.

       The   functions	 png_read_init(info_ptr),    png_write_init(info_ptr),
       png_info_init(info_ptr),	 png_read_destroy(),  and  png_write_destroy()
       have been removed.  They have been deprecated since libpng-0.95.

       The png_permit_empty_plte() was removed. It has been  deprecated	 since
       libpng-1.0.9.  Use png_permit_mng_features() instead.

       We   removed   the   obsolete  stub  functions  png_get_mmx_flagmask(),
       png_set_mmx_thresholds(),     png_get_asm_flags(),     png_get_mmx_bit‐
       depth_threshold(),		     png_get_mmx_rowbytes_threshold(),
       png_set_asm_flags(), and png_mmx_supported()

       We  removed  the	 obsolete  png_check_sig(),  png_memcpy_check(),   and
       png_memset_check()  functions.	Instead	 use !png_sig_cmp(), memcpy(),
       and memset(), respectively.

       The function png_set_gray_1_2_4_to_8() was removed. It has been	depre‐
       cated  since  libpng-1.0.18  and	 1.2.9,	 when  it  was	replaced  with
       png_set_expand_gray_1_2_4_to_8()	 because  the  former  function	  also
       expanded any tRNS chunk to an alpha channel.

       Macros  for  png_get_uint_16,  png_get_uint_32, and png_get_int_32 were
       added and are used by default instead of the  corresponding  functions.
       Unfortunately, from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro
       (but  not  the  function)  incorrectly  returned	 a   value   of	  type
       png_uint_32.

       We changed the prototype for png_malloc() from
	   png_malloc(png_structp png_ptr, png_uint_32 size) to
	   png_malloc(png_structp png_ptr, png_alloc_size_t size)

       This  also  applies  to	the  prototype	for  the user replacement mal‐
       loc_fn().

       The png_calloc() function  was  added  and  is  used  in	 place	of  of
       "png_malloc();  memset();"  except  in the case in png_read_png() where
       the array consists of pointers; in this case a "for" loop is used after
       the png_malloc() to set the pointers to NULL, to give robust.  behavior
       in case the  application	 runs  out  of	memory	part-way  through  the
       process.

       We  changed  the	 prototypes  of	 png_get_compression_buffer_size() and
       png_set_compression_buffer_size() to work with  png_size_t  instead  of
       png_uint_32.

       Support	for  numbered  error messages was removed by default, since we
       never got around to actually numbering the error messages. The function
       png_set_strip_error_numbers() was removed from the library by default.

       The png_zalloc() and png_zfree() functions are no longer exported.  The
       png_zalloc() function no longer zeroes out the  memory  that  it	 allo‐
       cates.	Applications that called png_zalloc(png_ptr, number, size) can
       call png_calloc(png_ptr, number*size) instead, and can call  png_free()
       instead of png_zfree().

       Support	for dithering was disabled by default in libpng-1.4.0, because
       it has not been well tested and doesn't actually	 "dither".   The  code
       was  not removed, however, and could be enabled by building libpng with
       PNG_READ_DITHER_SUPPORTED defined.  In libpng-1.4.2, this  support  was
       re-enabled,  but the function was renamed png_set_quantize() to reflect
       more  accurately	 what  it  actually  does.   At	 the  same  time,  the
       PNG_DITHER_[RED,GREEN_BLUE]_BITS	 macros were also renamed to PNG_QUAN‐
       TIZE_[RED,GREEN,BLUE]_BITS, and PNG_READ_DITHER_SUPPORTED  was  renamed
       to PNG_READ_QUANTIZE_SUPPORTED.

       We removed the trailing '.' from the warning and error messages.

XI. Changes to Libpng from version 1.4.x to 1.5.x
       From  libpng-1.4.0  until 1.4.4, the png_get_uint_16 macro (but not the
       function) incorrectly returned a value of type png_uint_32.  The incor‐
       rect macro was removed from libpng-1.4.5.

       Checking for invalid palette index on write was added at libpng 1.5.10.
       If a pixel contains an invalid (out-of-range)  index  libpng  issues  a
       benign  error.  This is enabled by default because this condition is an
       error according to the PNG specification, Clause 11.3.2, but the	 error
       can be ignored in each png_ptr with

	  png_set_check_for_invalid_index(png_ptr, allowed);

	     allowed  - one of
			0: disable benign error (accept the
			   invalid data without warning).
			1: enable benign error (treat the
			   invalid data as an error or a
			   warning).

       If  the error is ignored, or if png_benign_error() treats it as a warn‐
       ing, any invalid pixels are decoded as opaque black by the decoder  and
       written as-is by the encoder.

       Retrieving  the maximum palette index found was added at libpng-1.5.15.
       This statement must appear  after  png_read_png()  or  png_read_image()
       while  reading,	and  after  png_write_png() or png_write_image() while
       writing.

	  int max_palette = png_get_palette_max(png_ptr, info_ptr);

       This will return the maximum palette index found in the image, or  "-1"
       if  the	palette was not checked, or "0" if no palette was found.  Note
       that this does not account for any  palette  index  used	 by  ancillary
       chunks  such  as	 the  bKGD  chunk;  you must check those separately to
       determine the maximum palette index actually used.

       A. Changes that affect users of libpng

       There are no substantial API changes between the	 non-deprecated	 parts
       of  the	1.4.5  API and the 1.5.0 API; however, the ability to directly
       access members of the main libpng control  structures,  png_struct  and
       png_info, deprecated in earlier versions of libpng, has been completely
       removed from libpng 1.5.

       We no longer include zlib.h in png.h.  The include statement  has  been
       moved  to  pngstruct.h,	where  it  is  not accessible by applications.
       Applications that need access to information in zlib.h will need to add
       the  '#include "zlib.h"' directive.  It does not matter whether this is
       placed prior to or after the '"#include png.h"' directive.

       The png_sprintf(), png_strcpy(), and png_strncpy() macros are no longer
       used and were removed.

       We moved the png_strlen(), png_memcpy(), png_memset(), and png_memcmp()
       macros into a private header file (pngpriv.h) that is not accessible to
       applications.

       In  png_get_iCCP,  the type of "profile" was changed from png_charpp to
       png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep.

       There are changes of form in png.h, including new and changed macros to
       declare	parts  of the API.  Some API functions with arguments that are
       pointers to data not modified within the function have  been  corrected
       to declare these arguments with PNG_CONST.

       Much  of	 the internal use of C macros to control the library build has
       also changed and some of this is visible in the exported header	files,
       in  particular the use of macros to control data and API elements visi‐
       ble during application compilation may require significant revision  to
       application  code.   (It	 is  extremely	rare  for an application to do
       this.)

       Any program that compiled against libpng 1.4 and did not use deprecated
       features	 or access internal library structures should compile and work
       against libpng  1.5,  except  for  the  change  in  the	prototype  for
       png_get_iCCP() and png_set_iCCP() API functions mentioned above.

       libpng  1.5.0  adds PNG_ PASS macros to help in the reading and writing
       of interlaced images.  The macros return the number of rows and columns
       in  each	 pass and information that can be used to de-interlace and (if
       absolutely necessary) interlace an image.

       libpng 1.5.0 adds an API png_longjmp(png_ptr, value).  This  API	 calls
       the  application-provided png_longjmp_ptr on the internal, but applica‐
       tion initialized, longjmp buffer.  It is provided as a  convenience  to
       avoid  the  need to use the png_jmpbuf macro, which had the unnecessary
       side effect of resetting the internal png_longjmp_ptr value.

       libpng 1.5.0 includes a complete fixed point API.  By default  this  is
       present	along  with  the corresponding floating point API.  In general
       the fixed point API is faster and smaller than the floating  point  one
       because the PNG file format used fixed point, not floating point.  This
       applies even if the library uses floating point	in  internal  calcula‐
       tions.  A new macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether
       the library uses floating point arithmetic (the default) or fixed point
       arithmetic  internally  for  performance	 critical calculations such as
       gamma correction.  In some cases, the gamma  calculations  may  produce
       slightly	  different   results.	  This	has  changed  the  results  in
       png_rgb_to_gray and in alpha composition (png_set_background for	 exam‐
       ple). This applies even if the original image was already linear (gamma
       == 1.0) and, therefore, it is not necessary  to	linearize  the	image.
       This  is	 because  libpng  has *not* been changed to optimize that case
       correctly, yet.

       Fixed point support for the sCAL chunk comes with an important  caveat;
       the sCAL specification uses a decimal encoding of floating point values
       and the accuracy of PNG fixed point values is insufficient  for	repre‐
       sentation  of these values. Consequently a "string" API (png_get_sCAL_s
       and png_set_sCAL_s) is the only reliable way of reading arbitrary  sCAL
       chunks  in  the	absence	 of  either the floating point API or internal
       floating point calculations.  Starting with libpng-1.5.0, both of these
       functions  are  present	when  PNG_sCAL_SUPPORTED is defined.  Prior to
       libpng-1.5.0, their presence also  depended  upon  PNG_FIXED_POINT_SUP‐
       PORTED	being	defined	 and  PNG_FLOATING_POINT_SUPPORTED  not	 being
       defined.

       Applications no longer need to include the optional distribution header
       file  pngusr.h  or  define  the corresponding macros during application
       build in order to see the correct variant  of  the  libpng  API.	  From
       1.5.0  application  code	 can  check  for  the corresponding _SUPPORTED
       macro:

       #ifdef PNG_INCH_CONVERSIONS_SUPPORTED
	  /* code that uses the inch conversion APIs. */ #endif

       This macro will only be defined if the inch conversion  functions  have
       been  compiled into libpng.  The full set of macros, and whether or not
       support has been compiled in, are available in the header file  pnglib‐
       conf.h.	This header file is specific to the libpng build.  Notice that
       prior to 1.5.0 the _SUPPORTED macros would always have the default def‐
       inition	unless	reset  by pngusr.h or by explicit settings on the com‐
       piler command line.  These settings may produce	compiler  warnings  or
       errors in 1.5.0 because of macro redefinition.

       Applications  can now choose whether to use these macros or to call the
       corresponding   function	   by	 defining    PNG_USE_READ_MACROS    or
       PNG_NO_USE_READ_MACROS  before  including  png.h.   Notice that this is
       only supported from 1.5.0;  defining  PNG_NO_USE_READ_MACROS  prior  to
       1.5.0 will lead to a link failure.

       Prior to libpng-1.5.4, the zlib compressor used the same set of parame‐
       ters when compressing the IDAT data and textual data such as  zTXt  and
       iCCP.   In  libpng-1.5.4 we reinitialized the zlib stream for each type
       of data.	 We added five	png_set_text_*()  functions  for  setting  the
       parameters to use with textual data.

       Prior  to  libpng-1.5.4,	 the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
       option was off by default, and slightly	inaccurate  scaling  occurred.
       This  option can no longer be turned off, and the choice of accurate or
       inaccurate 16-to-8 scaling is by using the new  png_set_scale_16_to_8()
       API  for	 accurate  scaling  or the old png_set_strip_16_to_8() API for
       simple	chopping.    In	  libpng-1.5.4,	  the	PNG_READ_16_TO_8_ACCU‐
       RATE_SCALE_SUPPORTED macro became PNG_READ_SCALE_16_TO_8_SUPPORTED, and
       the PNG_READ_16_TO_8 macro became PNG_READ_STRIP_16_TO_8_SUPPORTED,  to
       enable the two png_set_*_16_to_8() functions separately.

       Prior to libpng-1.5.4, the png_set_user_limits() function could only be
       used  to	 reduce	 the  width  and  height  limits  from	the  value  of
       PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said
       that it could be used to override them.	Now this function will	reduce
       or increase the limits.

       Starting in libpng-1.5.10, the user limits can be set en masse with the
       configuration option  PNG_SAFE_LIMITS_SUPPORTED.	  If  this  option  is
       enabled,	 a set of "safe" limits is applied in pngpriv.h.  These can be
       overridden   by	  application	 calls	  to	png_set_user_limits(),
       png_set_user_chunk_cache_max(),	and/or	png_set_user_malloc_max() that
       increase or decrease the limits.	 Also, in  libpng-1.5.10  the  default
       width  and  height  limits  were	 increased from 1,000,000 to 0x7ffffff
       (i.e., made unlimited).	Therefore, the limits are now
				      default	   safe
	  png_user_width_max	    0x7fffffff	  1,000,000
	  png_user_height_max	    0x7fffffff	  1,000,000
	  png_user_chunk_cache_max  0 (unlimited)   128
	  png_user_chunk_malloc_max 0 (unlimited) 8,000,000

       The png_set_option() function (and the  "options"  member  of  the  png
       struct) was added to libpng-1.5.15.

       B. Changes to the build and configuration of libpng

       Details	of  internal  changes  to the library code can be found in the
       CHANGES file and in the GIT repository logs.  These will be of no  con‐
       cern  to	 the  vast majority of library users or builders; however, the
       few who configure libpng to a  non-default  feature  set	 may  need  to
       change how this is done.

       There  should be no need for library builders to alter build scripts if
       these use the distributed build support - configure or the makefiles  -
       however,	 users of the makefiles may care to update their build scripts
       to build pnglibconf.h where the corresponding makefile does not do so.

       Building libpng with  a	non-default  configuration  has	 changed  com‐
       pletely.	  The  old  method  using pngusr.h should still work correctly
       even though the way pngusr.h is used in the  build  has	been  changed;
       however,	 library builders will probably want to examine the changes to
       take advantage of new capabilities and to simplify their build system.

       B.1 Specific changes to library configuration capabilities

       The library now supports a complete fixed point implementation and  can
       thus  be	 used  on  systems that have no floating point support or very
       limited or slow support.	 Previously  gamma  correction,	 an  essential
       part of complete PNG support, required reasonably fast floating point.

       As  part	 of  this  the choice of internal implementation has been made
       independent of the choice of fixed versus floating point APIs  and  all
       the missing fixed point APIs have been implemented.

       The  exact  mechanism  used  to control attributes of API functions has
       changed.	 A single set of operating system  independent	macro  defini‐
       tions  is  used and operating system specific directives are defined in
       pnglibconf.h

       As part of this the mechanism used to choose procedure  call  standards
       on those systems that allow a choice has been changed.  At present this
       only affects certain Microsoft (DOS, Windows) and IBM (OS/2)  operating
       systems	running	 on  Intel  processors.	  As before, PNGAPI is defined
       where required to control the exported API functions; however, two  new
       macros,	PNGCBAPI  and PNGCAPI, are used instead for callback functions
       (PNGCBAPI) and (PNGCAPI) for functions that must match a C library pro‐
       totype  (currently only png_longjmp_ptr, which must match the C longjmp
       function.)  The new approach is documented in pngconf.h

       Despite these changes, libpng 1.5.0 only supports the native C function
       calling standard on those platforms tested so far (__cdecl on Microsoft
       Windows).  This is because the  support	requirements  for  alternative
       calling	conventions  seem  to no longer exist.	Developers who find it
       necessary to set PNG_API_RULE to 1 should advise the mailing list (png-
       mng-implement)  of  this	 and  library  builders who use Openwatcom and
       therefore set PNG_API_RULE to 2 should also contact the mailing list.

       A new test program, pngvalid, is provided in addition to pngtest.  png‐
       valid  validates the arithmetic accuracy of the gamma correction calcu‐
       lations and includes a number of validations of	the  file  format.   A
       subset  of the full range of tests is run when "make check" is done (in
       the 'configure' build.)	pngvalid also allows  total  allocated	memory
       usage  to be evaluated and performs additional memory overwrite valida‐
       tion.

       Many changes to individual feature macros have been made. The following
       are  the changes most likely to be noticed by library builders who con‐
       figure libpng:

       1) All feature macros now have consistent naming:

       #define PNG_NO_feature turns the feature off  #define  PNG_feature_SUP‐
       PORTED turns the feature on

       pnglibconf.h contains one line for each feature macro which is either:

       #define PNG_feature_SUPPORTED

       if the feature is supported or:

       /*#undef PNG_feature_SUPPORTED*/

       if  it  is  not.	  Library code consistently checks for the 'SUPPORTED'
       macro.  It does not, and libpng applications should not, check for  the
       'NO'  macro  which  will not normally be defined even if the feature is
       not supported.  The 'NO' macros are only used internally for setting or
       not setting the corresponding 'SUPPORTED' macros.

       Compatibility with the old names is provided as follows:

       PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED

       And the following definitions disable the corresponding feature:

       PNG_SETJMP_NOT_SUPPORTED	 disables  SETJMP PNG_READ_TRANSFORMS_NOT_SUP‐
       PORTED disables READ_TRANSFORMS	PNG_NO_READ_COMPOSITED_NODIV  disables
       READ_COMPOSITE_NODIV	PNG_WRITE_TRANSFORMS_NOT_SUPPORTED    disables
       WRITE_TRANSFORMS	   PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED    disables
       READ_ANCILLARY_CHUNKS PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables
       WRITE_ANCILLARY_CHUNKS

       Library builders should remove use of the above, inconsistent, names.

       2) Warning and error message formatting was previously  conditional  on
       the  STDIO  feature. The library has been changed to use the CONSOLE_IO
       feature instead. This means that if CONSOLE_IO is disabled the  library
       no  longer  uses	 the  printf(3)	 functions,  even  though  the default
       read/write implementations use (FILE) style stdio.h functions.

       3) Three feature macros now control the fixed/floating point decisions:

       PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs

       PNG_FIXED_POINT_SUPPORTED enables the fixed  point  APIs;  however,  in
       practice these are normally required internally anyway (because the PNG
       file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT
       merely stops the function from being exported.

       PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating
       point implementation or the fixed point one.  Typically the fixed point
       implementation is larger and slower than the floating point implementa‐
       tion on a system that supports  floating	 point;	 however,  it  may  be
       faster  on  a  system which lacks floating point hardware and therefore
       uses a software emulation.

       4) Added	 PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED.   This  allows  the
       functions  to  read  and	 write	ints  to  be disabled independently of
       PNG_USE_READ_MACROS, which allows libpng to be built with the functions
       even though the default is to use the macros - this allows applications
       to choose at app buildtime whether or not  to  use  macros  (previously
       impossible because the functions weren't in the default build.)

       B.2 Changes to the configuration mechanism

       Prior  to  libpng-1.5.0 library builders who needed to configure libpng
       had either to modify the exported pngconf.h header file to  add	system
       specific	 configuration	or  had to write feature selection macros into
       pngusr.h and cause this to  be  included	 into  pngconf.h  by  defining
       PNG_USER_CONFIG.	 The  latter  mechanism	 had  the disadvantage that an
       application built without PNG_USER_CONFIG defined would see the unmodi‐
       fied, default, libpng API and thus would probably fail to link.

       These  mechanisms still work in the configure build and in any makefile
       build that builds pnglibconf.h, although the feature  selection	macros
       have  changed somewhat as described above.  In 1.5.0, however, pngusr.h
       is processed only once, when the exported header file  pnglibconf.h  is
       built.	pngconf.h  no  longer includes pngusr.h, therefore pngusr.h is
       ignored after the build of pnglibconf.h and it is never included in  an
       application build.

       The  rarely  used alternative of adding a list of feature macros to the
       CPPFLAGS setting in the build also still	 works;	 however,  the	macros
       will  be copied to pnglibconf.h and this may produce macro redefinition
       warnings when the individual C files are compiled.

       All  configuration  now	only  works  if	 pnglibconf.h  is  built  from
       scripts/pnglibconf.dfa.	  This	 requires   the	 program  awk.	 Brian
       Kernighan (the original author of awk) maintains C source code of  that
       awk  and this and all known later implementations (often called by sub‐
       tly different names - nawk and gawk for example) are adequate to	 build
       pnglibconf.h.   The  Sun	 Microsystems (now Oracle) program 'awk' is an
       earlier version and does not work; this may also apply to other systems
       that have a functioning awk called 'nawk'.

       Configuration  options  are  now	 documented in scripts/pnglibconf.dfa.
       This file also includes dependency information that ensures a  configu‐
       ration  is  consistent; that is, if a feature is switched off dependent
       features are also removed.  As a recommended alternative to using  fea‐
       ture  macros  in	 pngusr.h  a system builder may also define equivalent
       options in pngusr.dfa (or, indeed, any file) and add that to  the  con‐
       figuration by setting DFA_XTRA to the file name.	 The makefiles in con‐
       trib/pngminim illustrate how to do this, and a case where  pngusr.h  is
       still required.

XII. Changes to Libpng from version 1.5.x to 1.6.x
       A  "simplified  API"  has  been added (see documentation in png.h and a
       simple example in contrib/examples/pngtopng.c).	The new publicly visi‐
       ble API includes the following:

	  macros:
	    PNG_FORMAT_*
	    PNG_IMAGE_*
	  structures:
	    png_control
	    png_image
	  read functions
	    png_image_begin_read_from_file()
	    png_image_begin_read_from_stdio()
	    png_image_begin_read_from_memory()
	    png_image_finish_read()
	    png_image_free()
	  write functions
	    png_image_write_to_file()
	    png_image_write_to_stdio()

       Starting	 with  libpng-1.6.0,  you  can	configure libpng to prefix all
       exported symbols, using the PNG_PREFIX macro.

       We no longer include string.h in png.h.	The include statement has been
       moved to pngpriv.h, where it is not accessible by applications.	Appli‐
       cations that need  access  to  information  in  string.h	 must  add  an
       '#include  <string.h>'  directive.   It does not matter whether this is
       placed prior to or after the '#include "png.h"' directive.

       The following API are now DEPRECATED:
	  png_info_init_3()
	  png_convert_to_rfc1123() which has been replaced
	    with png_convert_to_rfc1123_buffer()
	  png_malloc_default()
	  png_free_default()
	  png_reset_zstream()

       The following have been removed:
	  png_get_io_chunk_name(), which has been replaced
	    with png_get_io_chunk_type().  The new
	    function returns a 32-bit integer instead of
	    a string.
	  The png_sizeof(), png_strlen(), png_memcpy(), png_memcmp(), and
	    png_memset() macros are no longer used in the libpng sources and
	    have been removed.	These  had  already  been  made	 invisible  to
       applications
	    (i.e.,  defined  in	 the  private  pngpriv.h  header  file)	 since
       libpng-1.5.0.

       The signatures of many exported functions were changed, such that
	  png_structp became png_structrp or png_const_structrp
	  png_infop became png_inforp or png_const_inforp where "rp" indicates
       a "restricted pointer".

       Error  detection	 in  some  chunks has improved; in particular the iCCP
       chunk reader now does pretty complete validation of the	basic  format.
       Some bad profiles that were previously accepted are now accepted with a
       warning or rejected, depending upon  the	 png_set_benign_errors()  set‐
       ting,  in  particular  the  very old broken Microsoft/HP 3144-byte sRGB
       profile.	 The PNG spec requirement that	only  grayscale	 profiles  may
       appear in images with color type 0 or 4 and that even if the image only
       contains gray pixels, only RGB profiles may appear in images with color
       type  2, 3, or 6, is now enforced.  The sRGB chunk is allowed to appear
       in images with any color type.

       Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk  con‐
       tained an empty language field or an empty translated keyword.  Both of
       these are allowed by the PNG specification, so these  warnings  are  no
       longer issued.

       The  library  now  issues an error if the application attempts to set a
       transform after it calls png_read_update_info() or if  it  attempts  to
       call  both png_read_update_info() and png_start_read_image() or to call
       either of them more than once.

       The default condition for benign_errors is now to treat	benign	errors
       as warnings while reading and as errors while writing.

       The  library now issues a warning if both background processing and RGB
       to gray are used when gamma correction happens. As with	previous  ver‐
       sions of the library the results are numerically very incorrect in this
       case.

       There are some minor arithmetic changes	in  some  transforms  such  as
       png_set_background(),  that  might  be  detected	 by certain regression
       tests.

       Unknown chunk handling has been improved internally,  without  any  API
       change.	This adds more correct option control of the unknown handling,
       corrects a pre-existing bug  where  the	per-chunk  'keep'  setting  is
       ignored,	 and  makes  it possible to skip IDAT chunks in the sequential
       reader.

       The  machine-generated  configure  files	 are  no  longer  included  in
       branches libpng16 and later of the GIT repository.  They continue to be
       included in the tarball releases, however.

       Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning  of  the
       IDAT  stream  to set the size of the sliding window for reading instead
       of using the default 32-kbyte sliding window size.  It  was  discovered
       that  there  are	 hundreds of PNG files in the wild that have incorrect
       CMF bytes that caused libpng to issue a "too far back" error and reject
       the file.  Libpng-1.6.3 and later calculate their own safe CMF from the
       image dimensions, provide a way to revert to the libpng-1.5.x  behavior
       (ignoring the CMF bytes and using a 32-kbyte sliding window), by using

	   png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW,
	       PNG_OPTION_ON);

       and  provide a tool (contrib/tools/pngfix) for optimizing the CMF bytes
       correctly.

       Libpng-1.6.0 and libpng-1.6.1 wrote uncompressed iTXt chunks  with  the
       wrong  length,  which  resulted in PNG files that cannot be read beyond
       the bad iTXt chunk.  This error was fixed in libpng-1.6.3, and  a  tool
       (called	contrib/tools/png-fix-itxt)  has been added to the libpng dis‐
       tribution.

XIII. Detecting libpng
       The png_get_io_ptr() function has been present since  libpng-0.88,  has
       never changed, and is unaffected by conditional compilation macros.  It
       is the best choice for use in configure scripts for detecting the pres‐
       ence  of	 any libpng version since 0.88.	 In an autoconf "configure.in"
       you could use

	   AC_CHECK_LIB(png, png_get_io_ptr, ...

XV. Source code repository
       Since about February 2009, version 1.2.34, libpng has been under	 "git"
       source  control.	  The  git  repository	was  built  from  old  libpng-
       x.y.z.tar.gz files going back to version 0.70.  You can access the  git
       repository (read only) at

	   git://git.code.sf.net/p/libpng/code

       or  you can browse it with a web browser by selecting the "code" button
       at

	   https://sourceforge.net/projects/libpng

       Patches can be sent to glennrp at users.sourceforge.net or to  png-mng-
       implement at lists.sourceforge.net or you can upload them to the libpng
       bug tracker at

	   http://libpng.sourceforge.net

       We also accept patches built from the tar  or  zip  distributions,  and
       simple verbal discriptions of bug fixes, reported either to the Source‐
       Forge bug tracker, to the  png-mng-implement  at	 lists.sf.net  mailing
       list, or directly to glennrp.

XV. Coding style
       Our coding style is similar to the "Allman" style, with curly braces on
       separate lines:

	   if (condition)
	   {
	      action;
	   }

	   else if (another condition)
	   {
	      another action;
	   }

       The braces can be omitted from simple one-line actions:

	   if (condition)
	      return (0);

       We use 3-space indentation, except for continued statements  which  are
       usually	indented the same as the first line of the statement plus four
       more spaces.

       For macro definitions we use 2-space indentation,  always  leaving  the
       "#" in the first column.

	   #ifndef PNG_NO_FEATURE
	   #  ifndef PNG_FEATURE_SUPPORTED
	   #	define PNG_FEATURE_SUPPORTED
	   #  endif
	   #endif

       Comments	 appear	 with  the leading "/*" at the same indentation as the
       statement that follows the comment:

	   /* Single-line comment */
	   statement;

	   /* This is a multiple-line
	    * comment.
	    */
	   statement;

       Very short comments can be placed after the end	of  the	 statement  to
       which they pertain:

	   statement;	 /* comment */

       We  don't use C++ style ("//") comments. We have, however, used them in
       the past in some now-abandoned MMX assembler code.

       Functions and their curly braces are not indented, and  exported	 func‐
       tions are marked with PNGAPI:

	/* This is a public function that is visible to
	 * application programmers. It does thus-and-so.
	 */
	void PNGAPI
	png_exported_function(png_ptr, png_info, foo)
	{
	   body;
	}

       The  return type and decorations are placed on a separate line ahead of
       the function name, as illustrated above.

       The prototypes for all exported functions appear in  png.h,  above  the
       comment that says

	   /* Maintainer: Put new public prototypes here ... */

       We mark all non-exported functions with "/* PRIVATE */"":

	void /* PRIVATE */
	png_non_exported_function(png_ptr, png_info, foo)
	{
	   body;
	}

       The prototypes for non-exported functions (except for those in pngtest)
       appear in pngpriv.h above the comment that says

	 /* Maintainer: Put new private prototypes here ^ */

       We put a space after the "sizeof" operator and  we  omit	 the  optional
       parentheses around its argument when the argument is an expression, not
       a type name, and we always enclose the sizeof operator, with its	 argu‐
       ment, in parentheses:

	 (sizeof (png_uint_32))
	 (sizeof array)

       Prior  to  libpng-1.6.0	we  used  a "png_sizeof()" macro, formatted as
       though it were a function.

       To avoid polluting the global namespace,	 the  names  of	 all  exported
       functions  and  variables begin with "png_", and all publicly visible C
       preprocessor macros begin with "PNG".   We  request  that  applications
       that  use  libpng  *not*	 begin any of their own symbols with either of
       these strings.

       We put a space after each comma	and  after  each  semicolon  in	 "for"
       statements,  and	 we put spaces before and after each C binary operator
       and after "for" or "while", and before  "?".   We  don't	 put  a	 space
       between	a  typecast  and  the expression being cast, nor do we put one
       between a function name and the left parenthesis that follows it:

	   for (i = 2; i > 0; --i)
	      y[i] = a(x) + (int)b;

       We prefer #ifdef and #ifndef to #if defined() and #if  !defined()  when
       there  is  only one macro being tested.	We always use parentheses with
       "defined".

       We prefer to express integers that are used as bit masks in hex format,
       with  an	 even  number  of  lower-case  hex  digits  (e.g., 0x00, 0xff,
       0x0100).

       We prefer to use underscores in variable names rather  than  camelCase,
       except for a few type names that we inherit from zlib.h.

       We do not use the TAB character for indentation in the C sources.

       Lines do not exceed 80 characters.

       Other rules can be inferred by inspecting the libpng source.

XVI. Y2K Compliance in libpng
       March 6, 2014

       Since  the  PNG	Development  group is an ad-hoc body, we can't make an
       official declaration.

       This is your unofficial assurance that libpng  from  version  0.71  and
       upward  through 1.6.10 are Y2K compliant.  It is my belief that earlier
       versions were also Y2K compliant.

       Libpng only has two year fields.	 One is a 2-byte unsigned integer that
       will hold years up to 65535.  The other, which is deprecated, holds the
       date in text format, and will hold years up to 9999.

       The integer is
	   "png_uint_16 year" in png_time_struct.

       The string is
	   "char time_buffer[29]" in png_struct.  This is no  longer  used  in
       libpng-1.6.x and will be removed from libpng-1.7.0.

       There are seven time-related functions:

	   png_convert_to_rfc_1123() in png.c
	     (formerly png_convert_to_rfc_1152() in error)
	   png_convert_from_struct_tm() in pngwrite.c, called
	     in pngwrite.c
	   png_convert_from_time_t() in pngwrite.c
	   png_get_tIME() in pngget.c
	   png_handle_tIME() in pngrutil.c, called in pngread.c
	   png_set_tIME() in pngset.c
	   png_write_tIME() in pngwutil.c, called in pngwrite.c

       All appear to handle dates properly in a Y2K environment.  The png_con‐
       vert_from_time_t() function calls gmtime() to convert from system clock
       time,  which  returns  (year  - 1900), which we properly convert to the
       full 4-digit year.  There is  a	possibility  that  applications	 using
       libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
       function, or that they are incorrectly  passing	only  a	 2-digit  year
       instead	of  "year  - 1900" into the png_convert_from_struct_tm() func‐
       tion, but this is not under our control.	 The libpng documentation  has
       always  stated that it works with 4-digit years, and the APIs have been
       documented as such.

       The tIME chunk itself is also Y2K compliant.  It uses a 2-byte unsigned
       integer to hold the year, and can hold years as large as 65535.

       zlib, upon which libpng depends, is also Y2K compliant.	It contains no
       date-related code.

	  Glenn Randers-Pehrson
	  libpng maintainer
	  PNG Development Group

NOTE
       Note about libpng version numbers:

       Due to various miscommunications, unforeseen code incompatibilities and
       occasional  factors  outside the authors' control, version numbering on
       the library has not always been consistent  and	straightforward.   The
       following  table	 summarizes matters since version 0.89c, which was the
       first widely used release:

	source		   png.h  png.h	 shared-lib
	version		   string   int	 version
	-------		   ------  ----- ----------
	0.89c ("beta 3")  0.89	     89	 1.0.89
	0.90  ("beta 4")  0.90	     90	 0.90
	0.95  ("beta 5")  0.95	     95	 0.95
	0.96  ("beta 6")  0.96	     96	 0.96
	0.97b ("beta 7")  1.00.97    97	 1.0.1
	0.97c		  0.97	     97	 2.0.97
	0.98		  0.98	     98	 2.0.98
	0.99		  0.99	     98	 2.0.99
	0.99a-m		  0.99	     99	 2.0.99
	1.00		  1.00	    100	 2.1.0
	1.0.0		  1.0.0	    100	 2.1.0
	1.0.0	(from here on, the  100	 2.1.0
	1.0.1	 png.h string is  10001	 2.1.0
	1.0.1a-e identical to the 10002	 from here on, the
	1.0.2	 source version)  10002	 shared library is 2.V
	1.0.2a-b		  10003	 where V is the source
	1.0.1			  10001	 code version except as
	1.0.1a-e		  10002	 2.1.0.1a-e   noted.
	1.0.2			  10002	 2.1.0.2
	1.0.2a-b		  10003	 2.1.0.2a-b
	1.0.3			  10003	 2.1.0.3
	1.0.3a-d		  10004	 2.1.0.3a-d
	1.0.4			  10004	 2.1.0.4
	1.0.4a-f		  10005	 2.1.0.4a-f
	1.0.5 (+ 2 patches)	  10005	 2.1.0.5
	1.0.5a-d		  10006	 2.1.0.5a-d
	1.0.5e-r		  10100	 2.1.0.5e-r
	1.0.5s-v		  10006	 2.1.0.5s-v
	1.0.6 (+ 3 patches)	  10006	 2.1.0.6
	1.0.6d-g		  10007	 2.1.0.6d-g
	1.0.6h			  10007	 10.6h
	1.0.6i			  10007	 10.6i
	1.0.6j			  10007	 2.1.0.6j
	1.0.7beta11-14	  DLLNUM  10007	 2.1.0.7beta11-14
	1.0.7beta15-18	     1	  10007	 2.1.0.7beta15-18
	1.0.7rc1-2	     1	  10007	 2.1.0.7rc1-2
	1.0.7		     1	  10007	 2.1.0.7
	1.0.8beta1-4	     1	  10008	 2.1.0.8beta1-4
	1.0.8rc1	     1	  10008	 2.1.0.8rc1
	1.0.8		     1	  10008	 2.1.0.8
	1.0.9beta1-6	     1	  10009	 2.1.0.9beta1-6
	1.0.9rc1	     1	  10009	 2.1.0.9rc1
	1.0.9beta7-10	     1	  10009	 2.1.0.9beta7-10
	1.0.9rc2	     1	  10009	 2.1.0.9rc2
	1.0.9		     1	  10009	 2.1.0.9
	1.0.10beta1	     1	  10010	 2.1.0.10beta1
	1.0.10rc1	     1	  10010	 2.1.0.10rc1
	1.0.10		     1	  10010	 2.1.0.10
	1.0.11beta1-3	     1	  10011	 2.1.0.11beta1-3
	1.0.11rc1	     1	  10011	 2.1.0.11rc1
	1.0.11		     1	  10011	 2.1.0.11
	1.0.12beta1-2	     2	  10012	 2.1.0.12beta1-2
	1.0.12rc1	     2	  10012	 2.1.0.12rc1
	1.0.12		     2	  10012	 2.1.0.12
	1.1.0a-f	     -	  10100	 2.1.1.0a-f abandoned
	1.2.0beta1-2	     2	  10200	 2.1.2.0beta1-2
	1.2.0beta3-5	     3	  10200	 3.1.2.0beta3-5
	1.2.0rc1	     3	  10200	 3.1.2.0rc1
	1.2.0		     3	  10200	 3.1.2.0
	1.2.1beta-4	     3	  10201	 3.1.2.1beta1-4
	1.2.1rc1-2	     3	  10201	 3.1.2.1rc1-2
	1.2.1		     3	  10201	 3.1.2.1
	1.2.2beta1-6	    12	  10202	 12.so.0.1.2.2beta1-6
	1.0.13beta1	    10	  10013	 10.so.0.1.0.13beta1
	1.0.13rc1	    10	  10013	 10.so.0.1.0.13rc1
	1.2.2rc1	    12	  10202	 12.so.0.1.2.2rc1
	1.0.13		    10	  10013	 10.so.0.1.0.13
	1.2.2		    12	  10202	 12.so.0.1.2.2
	1.2.3rc1-6	    12	  10203	 12.so.0.1.2.3rc1-6
	1.2.3		    12	  10203	 12.so.0.1.2.3
	1.2.4beta1-3	    13	  10204	 12.so.0.1.2.4beta1-3
	1.2.4rc1	    13	  10204	 12.so.0.1.2.4rc1
	1.0.14		    10	  10014	 10.so.0.1.0.14
	1.2.4		    13	  10204	 12.so.0.1.2.4
	1.2.5beta1-2	    13	  10205	 12.so.0.1.2.5beta1-2
	1.0.15rc1	    10	  10015	 10.so.0.1.0.15rc1
	1.0.15		    10	  10015	 10.so.0.1.0.15
	1.2.5		    13	  10205	 12.so.0.1.2.5
	1.2.6beta1-4	    13	  10206	 12.so.0.1.2.6beta1-4
	1.2.6rc1-5	    13	  10206	 12.so.0.1.2.6rc1-5
	1.0.16		    10	  10016	 10.so.0.1.0.16
	1.2.6		    13	  10206	 12.so.0.1.2.6
	1.2.7beta1-2	    13	  10207	 12.so.0.1.2.7beta1-2
	1.0.17rc1	    10	  10017	 12.so.0.1.0.17rc1
	1.2.7rc1	    13	  10207	 12.so.0.1.2.7rc1
	1.0.17		    10	  10017	 12.so.0.1.0.17
	1.2.7		    13	  10207	 12.so.0.1.2.7
	1.2.8beta1-5	    13	  10208	 12.so.0.1.2.8beta1-5
	1.0.18rc1-5	    10	  10018	 12.so.0.1.0.18rc1-5
	1.2.8rc1-5	    13	  10208	 12.so.0.1.2.8rc1-5
	1.0.18		    10	  10018	 12.so.0.1.0.18
	1.2.8		    13	  10208	 12.so.0.1.2.8
	1.2.9beta1-3	    13	  10209	 12.so.0.1.2.9beta1-3
	1.2.9beta4-11	    13	  10209	 12.so.0.9[.0]
	1.2.9rc1	    13	  10209	 12.so.0.9[.0]
	1.2.9		    13	  10209	 12.so.0.9[.0]
	1.2.10beta1-7	    13	  10210	 12.so.0.10[.0]
	1.2.10rc1-2	    13	  10210	 12.so.0.10[.0]
	1.2.10		    13	  10210	 12.so.0.10[.0]
	1.4.0beta1-6	    14	  10400	 14.so.0.0[.0]
	1.2.11beta1-4	    13	  10210	 12.so.0.11[.0]
	1.4.0beta7-8	    14	  10400	 14.so.0.0[.0]
	1.2.11		    13	  10211	 12.so.0.11[.0]
	1.2.12		    13	  10212	 12.so.0.12[.0]
	1.4.0beta9-14	    14	  10400	 14.so.0.0[.0]
	1.2.13		    13	  10213	 12.so.0.13[.0]
	1.4.0beta15-36	    14	  10400	 14.so.0.0[.0]
	1.4.0beta37-87	    14	  10400	 14.so.14.0[.0]
	1.4.0rc01	    14	  10400	 14.so.14.0[.0]
	1.4.0beta88-109	    14	  10400	 14.so.14.0[.0]
	1.4.0rc02-08	    14	  10400	 14.so.14.0[.0]
	1.4.0		    14	  10400	 14.so.14.0[.0]
	1.4.1beta01-03	    14	  10401	 14.so.14.1[.0]
	1.4.1rc01	    14	  10401	 14.so.14.1[.0]
	1.4.1beta04-12	    14	  10401	 14.so.14.1[.0]
	1.4.1		    14	  10401	 14.so.14.1[.0]
	1.4.2		    14	  10402	 14.so.14.2[.0]
	1.4.3		    14	  10403	 14.so.14.3[.0]
	1.4.4		    14	  10404	 14.so.14.4[.0]
	1.5.0beta01-58	    15	  10500	 15.so.15.0[.0]
	1.5.0rc01-07	    15	  10500	 15.so.15.0[.0]
	1.5.0		    15	  10500	 15.so.15.0[.0]
	1.5.1beta01-11	    15	  10501	 15.so.15.1[.0]
	1.5.1rc01-02	    15	  10501	 15.so.15.1[.0]
	1.5.1		    15	  10501	 15.so.15.1[.0]
	1.5.2beta01-03	    15	  10502	 15.so.15.2[.0]
	1.5.2rc01-03	    15	  10502	 15.so.15.2[.0]
	1.5.2		    15	  10502	 15.so.15.2[.0]
	1.5.3beta01-10	    15	  10503	 15.so.15.3[.0]
	1.5.3rc01-02	    15	  10503	 15.so.15.3[.0]
	1.5.3beta11	    15	  10503	 15.so.15.3[.0]
	1.5.3 [omitted]
	1.5.4beta01-08	    15	  10504	 15.so.15.4[.0]
	1.5.4rc01	    15	  10504	 15.so.15.4[.0]
	1.5.4		    15	  10504	 15.so.15.4[.0]
	1.5.5beta01-08	    15	  10505	 15.so.15.5[.0]
	1.5.5rc01	    15	  10505	 15.so.15.5[.0]
	1.5.5		    15	  10505	 15.so.15.5[.0]
	1.5.6beta01-07	    15	  10506	 15.so.15.6[.0]
	1.5.6rc01-03	    15	  10506	 15.so.15.6[.0]
	1.5.6		    15	  10506	 15.so.15.6[.0]
	1.5.7beta01-05	    15	  10507	 15.so.15.7[.0]
	1.5.7rc01-03	    15	  10507	 15.so.15.7[.0]
	1.5.7		    15	  10507	 15.so.15.7[.0]
	1.6.0beta01-40	    16	  10600	 16.so.16.0[.0]
	1.6.0rc01-08	    16	  10600	 16.so.16.0[.0]
	1.6.0		    16	  10600	 16.so.16.0[.0]
	1.6.1beta01-09	    16	  10601	 16.so.16.1[.0]
	1.6.1rc01	    16	  10601	 16.so.16.1[.0]
	1.6.1		    16	  10601	 16.so.16.1[.0]
	1.6.2beta01	    16	  10602	 16.so.16.2[.0]
	1.6.2rc01-06	    16	  10602	 16.so.16.2[.0]
	1.6.2		    16	  10602	 16.so.16.2[.0]
	1.6.3beta01-11	    16	  10603	 16.so.16.3[.0]
	1.6.3rc01	    16	  10603	 16.so.16.3[.0]
	1.6.3		    16	  10603	 16.so.16.3[.0]
	1.6.4beta01-02	    16	  10604	 16.so.16.4[.0]
	1.6.4rc01	    16	  10604	 16.so.16.4[.0]
	1.6.4		    16	  10604	 16.so.16.4[.0]
	1.6.5		    16	  10605	 16.so.16.5[.0]
	1.6.6		    16	  10606	 16.so.16.6[.0]
	1.6.7beta01-04	    16	  10607	 16.so.16.7[.0]
	1.6.7rc01-02	    16	  10607	 16.so.16.7[.0]
	1.6.7		    16	  10607	 16.so.16.7[.0]
	1.6.8beta01-02	    16	  10608	 16.so.16.8[.0]
	1.6.8rc01-02	    16	  10608	 16.so.16.8[.0]
	1.6.8		    16	  10608	 16.so.16.8[.0]
	1.6.9beta01-04	    16	  10609	 16.so.16.9[.0]
	1.6.9rc01-02	    16	  10609	 16.so.16.9[.0]
	1.6.9		    16	  10609	 16.so.16.9[.0]
	1.6.10beta01-03	    16	  10610	 16.so.16.10[.0]
	1.6.10rc01-04	    16	  10610	 16.so.16.10[.0]
	1.6.10		    16	  10610	 16.so.16.10[.0]

       Henceforth the source version will match the shared-library  minor  and
       patch numbers; the shared-library major version number will be used for
       changes	in  backward  compatibility,   as   it	 is   intended.	   The
       PNG_PNGLIB_VER  macro, which is not used within libpng but is available
       for applications, is an unsigned integer of the form xyyzz  correspond‐
       ing  to the source version x.y.z (leading zeros in y and z).  Beta ver‐
       sions were given the previous public  release  number  plus  a  letter,
       until  version 1.0.6j; from then on they were given the upcoming public
       release number plus "betaNN" or "rcN".

SEE ALSO
       png(5), libpngpf(3), zlib(3), deflate(5), and zlib(5)

       libpng:

	      http://libpng.sourceforge.net  (follow  the   [DOWNLOAD]	 link)
	      http://www.libpng.org/pub/png

       zlib:

	      (generally) at the same location as libpng or at
	      ftp://ftp.info-zip.org/pub/infozip/zlib

       PNGspecification:RFC2083

	      (generally) at the same location as libpng or at
	      ftp://ds.internic.net/rfc/rfc2083.txt
	      or (as a W3C Recommendation) at
	      http://www.w3.org/TR/REC-png.html

       In the case of any inconsistency between the PNG specification and this
       library, the specification takes precedence.

AUTHORS
       This man page: Glenn Randers-Pehrson <glennrp at users.sourceforge.net>

       The contributing authors would like to thank all those who helped  with
       testing,	 bug  fixes,  and  patience.  This wouldn't have been possible
       without all of you.

       Thanks to Frank J. T. Wojcik for helping with the documentation.

       Libpng version 1.6.10 - March 6, 2014: Initially created in 1995 by Guy
       Eric  Schalnat,	then  of Group 42, Inc.	 Currently maintained by Glenn
       Randers-Pehrson (glennrp at users.sourceforge.net).

       Supported by the PNG development group
       png-mng-implement at lists.sf.net (subscription	required;  visit  png-
       mng-implement  at  lists.sourceforge.net	 (subscription required; visit
       https://lists.sourceforge.net/lists/listinfo/png-mng-implement to  sub‐
       scribe).

COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:
       (This  copy of the libpng notices is provided for your convenience.  In
       case of any discrepancy between this copy and the notices in  the  file
       png.h  that  is	included  in the libpng distribution, the latter shall
       prevail.)

       If you modify libpng you may insert additional notices immediately fol‐
       lowing this sentence.

       This code is released under the libpng license.

       libpng  versions 1.2.6, August 15, 2004, through 1.6.10, March 6, 2014,
       are Copyright (c) 2004,2006-2007 Glenn Randers-Pehrson,	and  are  dis‐
       tributed	 according  to the same disclaimer and license as libpng-1.2.5
       with the following individual added to the list of Contributing Authors

	  Cosmin Truta

       libpng versions 1.0.7, July 1, 2000, through 1.2.5 - October  3,	 2002,
       are  Copyright (c) 2000-2002 Glenn Randers-Pehrson, and are distributed
       according to the same disclaimer and license as libpng-1.0.6  with  the
       following individuals added to the list of Contributing Authors

	  Simon-Pierre Cadieux
	  Eric S. Raymond
	  Gilles Vollant

       and with the following additions to the disclaimer:

	  There is no warranty against interference with your
	  enjoyment of the library or against infringement.
	  There is no warranty that our efforts or the library
	  will fulfill any of your particular purposes or needs.
	  This library is provided with all faults, and the entire
	  risk of satisfactory quality, performance, accuracy, and
	  effort is with the user.

       libpng  versions 0.97, January 1998, through 1.0.6, March 20, 2000, are
       Copyright (c) 1998, 1999 Glenn Randers-Pehrson Distributed according to
       the  same  disclaimer  and  license  as libpng-0.96, with the following
       individuals added to the list of Contributing Authors:

	  Tom Lane
	  Glenn Randers-Pehrson
	  Willem van Schaik

       libpng versions 0.89, June 1996, through 0.96, May 1997, are  Copyright
       (c)  1996,  1997	 Andreas Dilger Distributed according to the same dis‐
       claimer and license as  libpng-0.88,  with  the	following  individuals
       added to the list of Contributing Authors:

	  John Bowler
	  Kevin Bracey
	  Sam Bushell
	  Magnus Holmgren
	  Greg Roelofs
	  Tom Tanner

       libpng  versions	 0.5,  May 1995, through 0.88, January 1996, are Copy‐
       right (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.

       For the purposes of this copyright and license, "Contributing  Authors"
       is defined as the following set of individuals:

	  Andreas Dilger
	  Dave Martindale
	  Guy Eric Schalnat
	  Paul Schmidt
	  Tim Wegner

       The  PNG	 Reference  Library  is	 supplied  "AS	IS".  The Contributing
       Authors and Group  42,  Inc.  disclaim  all  warranties,	 expressed  or
       implied, including, without limitation, the warranties of merchantabil‐
       ity and of fitness for any purpose.  The Contributing Authors and Group
       42,  Inc.   assume  no liability for direct, indirect, incidental, spe‐
       cial, exemplary, or consequential damages, which may  result  from  the
       use of the PNG Reference Library, even if advised of the possibility of
       such damage.

       Permission is hereby granted to use, copy, modify, and distribute  this
       source  code, or portions hereof, for any purpose, without fee, subject
       to the following restrictions:

       1. The origin of this source code must not be misrepresented.

       2. Altered versions must be plainly marked as such and
	  must not be misrepresented as being the original source.

       3. This Copyright notice may not be removed or altered from
	  any source or altered source distribution.

       The Contributing Authors and Group 42, Inc. specifically permit,	 with‐
       out  fee,  and  encourage the use of this source code as a component to
       supporting the PNG file format in commercial products.  If you use this
       source  code  in a product, acknowledgment is not required but would be
       appreciated.

       A "png_get_copyright" function is  available,  for  convenient  use  in
       "about" boxes and the like:

	  printf("%s",png_get_copyright(NULL));

       Also,  the PNG logo (in PNG format, of course) is supplied in the files
       "pngbar.png" and "pngbar.jpg (88x31) and "pngnow.png" (98x31).

       Libpng is OSI Certified	Open  Source  Software.	  OSI  Certified  Open
       Source is a certification mark of the Open Source Initiative.

       Glenn Randers-Pehrson glennrp at users.sourceforge.net March 6, 2014

				 March 6, 2014			     LIBPNG(3)
[top]

List of man pages available for SmartOS

Copyright (c) for man pages and the logo by the respective OS vendor.

For those who want to learn more, the polarhome community provides shell access and support.

[legal] [privacy] [GNU] [policy] [cookies] [netiquette] [sponsors] [FAQ]
Tweet
Polarhome, production since 1999.
Member of Polarhome portal.
Based on Fawad Halim's script.
....................................................................
Vote for polarhome
Free Shell Accounts :: the biggest list on the net