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DJPEG(1)							      DJPEG(1)

NAME
       djpeg - decompress a JPEG file to an image file

SYNOPSIS
       djpeg [ options ] [ filename ]

DESCRIPTION
       djpeg  decompresses  the	 named	JPEG file, or the standard input if no
       file is named, and produces an image file on the standard output.  PBM‐
       PLUS  (PPM/PGM),	 BMP,  GIF, Targa, or RLE (Utah Raster Toolkit) output
       format can be selected.	(RLE is supported only if the URT  library  is
       available.)

OPTIONS
       All  switch  names  may	be abbreviated; for example, -grayscale may be
       written -gray or -gr.  Most of the "basic" switches can be  abbreviated
       to  as little as one letter.  Upper and lower case are equivalent (thus
       -BMP is the same as -bmp).  British spellings are also accepted	(e.g.,
       -greyscale), though for brevity these are not mentioned below.

       The basic switches are:

       -colors N
	      Reduce  image  to	 at most N colors.  This reduces the number of
	      colors used in the output image, so that it can be displayed  on
	      a	 colormapped  display  or stored in a colormapped file format.
	      For example, if you have an 8-bit display, you'd need to	reduce
	      to 256 or fewer colors.

       -quantize N
	      Same  as -colors.	 -colors is the recommended name, -quantize is
	      provided only for backwards compatibility.

       -fast  Select recommended processing options for fast, low quality out‐
	      put.   (The  default options are chosen for highest quality out‐
	      put.)  Currently, this is	 equivalent  to	 -dct  fast  -nosmooth
	      -onepass -dither ordered.

       -grayscale
	      Force  grayscale	output even if JPEG file is color.  Useful for
	      viewing on monochrome  displays;	also,  djpeg  runs  noticeably
	      faster in this mode.

       -rgb   Force RGB output even if JPEG file is grayscale.

       -scale M/N
	      Scale  the  output  image	 by a factor M/N.  Currently the scale
	      factor must be M/8, where M is  an  integer  between  1  and  16
	      inclusive,  or  any  reduced fraction thereof (such as 1/2, 3/4,
	      etc.)  Scaling is handy if the image is larger than your screen;
	      also, djpeg runs much faster when scaling down the output.

       -bmp   Select  BMP  output  format (Windows flavor).  8-bit colormapped
	      format is emitted if -colors or -grayscale is specified,	or  if
	      the  JPEG file is grayscale; otherwise, 24-bit full-color format
	      is emitted.

       -gif   Select GIF output format.	 Since GIF does not support more  than
	      256 colors, -colors 256 is assumed (unless you specify a smaller
	      number of colors).

       -os2   Select BMP output format (OS/2 1.x flavor).   8-bit  colormapped
	      format  is  emitted if -colors or -grayscale is specified, or if
	      the JPEG file is grayscale; otherwise, 24-bit full-color	format
	      is emitted.

       -pnm   Select PBMPLUS (PPM/PGM) output format (this is the default for‐
	      mat).  PGM is emitted if	the  JPEG  file	 is  grayscale	or  if
	      -grayscale is specified; otherwise PPM is emitted.

       -rle   Select RLE output format.	 (Requires URT library.)

       -targa Select  Targa output format.  Grayscale format is emitted if the
	      JPEG file is grayscale or if -grayscale is specified; otherwise,
	      colormapped  format  is  emitted if -colors is specified; other‐
	      wise, 24-bit full-color format is emitted.

       Switches for advanced users:

       -dct int
	      Use integer DCT method (default).

       -dct fast
	      Use fast integer DCT (less  accurate).   In  libjpeg-turbo,  the
	      fast  method is generally about 5-15% faster than the int method
	      when using the x86/x86-64 SIMD extensions (results may vary with
	      other  SIMD implementations, or when using libjpeg-turbo without
	      SIMD extensions.)	 If the JPEG  image  was  compressed  using  a
	      quality  level of 85 or below, then there should be little or no
	      perceptible difference between the two algorithms.  When	decom‐
	      pressing	images that were compressed using quality levels above
	      85, however, the difference between the  fast  and  int  methods
	      becomes  more  pronounced.   With	 images compressed using qual‐
	      ity=97, for instance, the fast method incurs generally  about  a
	      4-6  dB  loss (in PSNR) relative to the int method, but this can
	      be larger for some images.  If you can avoid it, do not use  the
	      fast method when decompressing images that were compressed using
	      quality levels above 97.	The algorithm  often  degenerates  for
	      such  images  and can actually produce a more lossy output image
	      than if the JPEG image had been compressed using	lower  quality
	      levels.

       -dct float
	      Use  floating-point  DCT	method.	  The float method is mainly a
	      legacy feature.  It does not produce significantly more accurate
	      results  than  the int method, and it is much slower.  The float
	      method may also give different results on different machines due
	      to varying roundoff behavior, whereas the integer methods should
	      give the same results on all machines.

       -dither fs
	      Use Floyd-Steinberg dithering in color quantization.

       -dither ordered
	      Use ordered dithering in color quantization.

       -dither none
	      Do not use dithering in color quantization.  By default,	Floyd-
	      Steinberg	 dithering  is applied when quantizing colors; this is
	      slow but usually produces the best results.  Ordered dither is a
	      compromise  between  speed and quality; no dithering is fast but
	      usually looks awful.  Note that these switches  have  no	effect
	      unless color quantization is being done.	Ordered dither is only
	      available in -onepass mode.

       -map file
	      Quantize to the colors used in the specified image  file.	  This
	      is  useful  for  producing  multiple  files with identical color
	      maps, or for forcing a predefined set of colors to be used.  The
	      file  must  be  a GIF or PPM file. This option overrides -colors
	      and -onepass.

       -nosmooth
	      Use a faster, lower-quality upsampling routine.

       -onepass
	      Use one-pass instead of two-pass color quantization.   The  one-
	      pass  method  is faster and needs less memory, but it produces a
	      lower-quality image.  -onepass is ignored unless	you  also  say
	      -colors  N.   Also,  the	one-pass  method  is  always  used for
	      grayscale output (the two-pass method is no improvement then).

       -maxmemory N
	      Set limit for amount  of	memory	to  use	 in  processing	 large
	      images.  Value is in thousands of bytes, or millions of bytes if
	      "M" is attached to the number.  For  example,  -max  4m  selects
	      4000000 bytes.  If more space is needed, an error will occur.

       -outfile name
	      Send output image to the named file, not to standard output.

       -memsrc
	      Load  input file into memory before decompressing.  This feature
	      was implemented mainly as a way of testing the in-memory	source
	      manager (jpeg_mem_src().)

       -skip Y0,Y1
	      Decompress  all  rows  of the JPEG image except those between Y0
	      and Y1 (inclusive.)  Note that if decompression scaling is being
	      used,  then  Y0  and  Y1 are relative to the scaled image dimen‐
	      sions.

       -crop WxH+X+Y
	      Decompress only a rectangular subregion of the  image,  starting
	      at point X,Y with width W and height H.  If necessary, X will be
	      shifted left to the nearest iMCU boundary, and the width will be
	      increased	 accordingly.	Note  that if decompression scaling is
	      being used, then X, Y, W, and H are relative to the scaled image
	      dimensions.

       -verbose
	      Enable  debug printout.  More -v's give more output.  Also, ver‐
	      sion information is printed at startup.

       -debug Same as -verbose.

       -version
	      Print version information and exit.

EXAMPLES
       This example decompresses the JPEG file foo.jpg, quantizes  it  to  256
       colors, and saves the output in 8-bit BMP format in foo.bmp:

	      djpeg -colors 256 -bmp foo.jpg > foo.bmp

HINTS
       To  get	a  quick preview of an image, use the -grayscale and/or -scale
       switches.  -grayscale -scale 1/8 is the fastest case.

       Several options are available that trade	 off  image  quality  to  gain
       speed.  -fast turns on the recommended settings.

       -dct  fast and/or -nosmooth gain speed at a small sacrifice in quality.
       When producing a color-quantized image,	-onepass  -dither  ordered  is
       fast  but  much	lower quality than the default behavior.  -dither none
       may give acceptable results in two-pass mode, but is  seldom  tolerable
       in one-pass mode.

       If  you are fortunate enough to have very fast floating point hardware,
       -dct float may be even faster than -dct fast.   But  on	most  machines
       -dct float is slower than -dct int; in this case it is not worth using,
       because its theoretical accuracy advantage is too small to be  signifi‐
       cant in practice.

ENVIRONMENT
       JPEGMEM
	      If  this	environment  variable is set, its value is the default
	      memory limit.  The value	is  specified  as  described  for  the
	      -maxmemory  switch.   JPEGMEM overrides the default value speci‐
	      fied when the program was compiled, and itself is overridden  by
	      an explicit -maxmemory.

SEE ALSO
       cjpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1)
       ppm(5), pgm(5)
       Wallace,	 Gregory  K.   "The  JPEG Still Picture Compression Standard",
       Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.

AUTHOR
       Independent JPEG Group

       This file was modified by The libjpeg-turbo  Project  to	 include  only
       information  relevant  to libjpeg-turbo, to wordsmith certain sections,
       and to describe features not present in libjpeg.

ISSUES
       Support for compressed GIF output files was removed in djpeg v6b due to
       concerns	 over  the Unisys LZW patent.  Although this patent expired in
       2006, djpeg still lacks compressed GIF support,	for  these  historical
       reasons.	  (Conversion  of JPEG files to GIF is usually a bad idea any‐
       way, since GIF is a 256-color format.)  The uncompressed GIF files that
       djpeg  generates	 are larger than they should be, but they are readable
       by standard GIF decoders.

				 18 March 2017			      DJPEG(1)
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