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GLREADPIXELS(3G)		 OpenGL Manual		      GLREADPIXELS(3G)

NAME
       glReadPixels - read a block of pixels from the frame buffer

C SPECIFICATION
       void glReadPixels(GLint x, GLint y, GLsizei width, GLsizei height,
			 GLenum format, GLenum type, GLvoid * data);

PARAMETERS
       x, y
	   Specify the window coordinates of the first pixel that is read from
	   the frame buffer. This location is the lower left corner of a
	   rectangular block of pixels.

       width, height
	   Specify the dimensions of the pixel rectangle.  width and height of
	   one correspond to a single pixel.

       format
	   Specifies the format of the pixel data. The following symbolic
	   values are accepted: GL_STENCIL_INDEX, GL_DEPTH_COMPONENT,
	   GL_DEPTH_STENCIL, GL_RED, GL_GREEN, GL_BLUE, GL_RGB, GL_BGR,
	   GL_RGBA, and GL_BGRA.

       type
	   Specifies the data type of the pixel data. Must be one of
	   GL_UNSIGNED_BYTE, GL_BYTE, GL_UNSIGNED_SHORT, GL_SHORT,
	   GL_UNSIGNED_INT, GL_INT, GL_HALF_FLOAT, GL_FLOAT,
	   GL_UNSIGNED_BYTE_3_3_2, GL_UNSIGNED_BYTE_2_3_3_REV,
	   GL_UNSIGNED_SHORT_5_6_5, GL_UNSIGNED_SHORT_5_6_5_REV,
	   GL_UNSIGNED_SHORT_4_4_4_4, GL_UNSIGNED_SHORT_4_4_4_4_REV,
	   GL_UNSIGNED_SHORT_5_5_5_1, GL_UNSIGNED_SHORT_1_5_5_5_REV,
	   GL_UNSIGNED_INT_8_8_8_8, GL_UNSIGNED_INT_8_8_8_8_REV,
	   GL_UNSIGNED_INT_10_10_10_2, GL_UNSIGNED_INT_2_10_10_10_REV,
	   GL_UNSIGNED_INT_24_8, GL_UNSIGNED_INT_10F_11F_11F_REV,
	   GL_UNSIGNED_INT_5_9_9_9_REV, or GL_FLOAT_32_UNSIGNED_INT_24_8_REV.

       data
	   Returns the pixel data.

DESCRIPTION
       glReadPixels returns pixel data from the frame buffer, starting with
       the pixel whose lower left corner is at location (x, y), into client
       memory starting at location data. Several parameters control the
       processing of the pixel data before it is placed into client memory.
       These parameters are set with glPixelStore(). This reference page
       describes the effects on glReadPixels of most, but not all of the
       parameters specified by these three commands.

       If a non-zero named buffer object is bound to the GL_PIXEL_PACK_BUFFER
       target (see glBindBuffer()) while a block of pixels is requested, data
       is treated as a byte offset into the buffer object's data store rather
       than a pointer to client memory.

       glReadPixels returns values from each pixel with lower left corner at x
       + i y + j for 0 <= i < width and 0 <= j < height. This pixel is said to
       be the ith pixel in the jth row. Pixels are returned in row order from
       the lowest to the highest row, left to right in each row.

       format specifies the format for the returned pixel values; accepted
       values are:

       GL_STENCIL_INDEX
	   Stencil values are read from the stencil buffer. Each index is
	   converted to fixed point, shifted left or right depending on the
	   value and sign of GL_INDEX_SHIFT, and added to GL_INDEX_OFFSET. If
	   GL_MAP_STENCIL is GL_TRUE, indices are replaced by their mappings
	   in the table GL_PIXEL_MAP_S_TO_S.

       GL_DEPTH_COMPONENT
	   Depth values are read from the depth buffer. Each component is
	   converted to floating point such that the minimum depth value maps
	   to 0 and the maximum value maps to 1. Each component is then
	   multiplied by GL_DEPTH_SCALE, added to GL_DEPTH_BIAS, and finally
	   clamped to the range 0 1.

       GL_DEPTH_STENCIL
	   Values are taken from both the depth and stencil buffers. The type
	   parameter must be GL_UNSIGNED_INT_24_8 or
	   GL_FLOAT_32_UNSIGNED_INT_24_8_REV.

       GL_RED

       GL_GREEN

       GL_BLUE

       GL_RGB

       GL_BGR

       GL_RGBA

       GL_BGRA

       Finally, the indices or components are converted to the proper format,
       as specified by type. If format is GL_STENCIL_INDEX and type is not
       GL_FLOAT, each index is masked with the mask value given in the
       following table. If type is GL_FLOAT, then each integer index is
       converted to single-precision floating-point format.

       If format is GL_RED, GL_GREEN, GL_BLUE, GL_RGB, GL_BGR, GL_RGBA, or
       GL_BGRA and type is not GL_FLOAT, each component is multiplied by the
       multiplier shown in the following table. If type is GL_FLOAT, then each
       component is passed as is (or converted to the client's
       single-precision floating-point format if it is different from the one
       used by the GL).

       ┌──────────────────────────────────┬────────────────────────────────────┬────────────────────────────────────────────────────┐
       │type				  │				       │						    │
       │				  │			      Index    │				 Component	    │
       │				  │   Mask			       │	 Conversion				    │
       │				  │				       │						    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_BYTE		  │ 2				       │     2						    │
       │				  │				    8  │					     8	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_BYTE				  │ 2				       │ 2						    │
       │				  │				    7  │						 8  │
       │				  │				       │						    │
       │				  │				    -  │						 -  │
       │				  │				    1  │						 1  │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │					 ⁢	   │
       │				  │				       │					 c	    │
       │				  │				       │					 -	    │
       │				  │				       │					 1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				 2		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_SHORT		  │ 2				       │     2						    │
       │				  │				    16 │					     16	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_SHORT			  │ 2				       │ 2						    │
       │				  │				    15 │						 16 │
       │				  │				       │						    │
       │				  │				    -  │						 -  │
       │				  │				    1  │						 1  │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │					 ⁢	   │
       │				  │				       │					 c	    │
       │				  │				       │					 -	    │
       │				  │				       │					 1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				 2		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_INT			  │ 2				       │     2						    │
       │				  │				    32 │					     32	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_INT				  │ 2				       │ 2						    │
       │				  │				    31 │						 32 │
       │				  │				       │						    │
       │				  │				    -  │						 -  │
       │				  │				    1  │						 1  │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │					 ⁢	   │
       │				  │				       │					 c	    │
       │				  │				       │					 -	    │
       │				  │				       │					 1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				 2		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_HALF_FLOAT			  │		   none		       │			 c			    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_FLOAT			  │		   none		       │			 c			    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_BYTE_3_3_2		  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_BYTE_2_3_3_REV	  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_SHORT_5_6_5		  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_SHORT_5_6_5_REV	  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_SHORT_4_4_4_4	  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_SHORT_4_4_4_4_REV	  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_SHORT_5_5_5_1	  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_SHORT_1_5_5_5_REV	  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_INT_8_8_8_8		  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_INT_8_8_8_8_REV	  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_INT_10_10_10_2	  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_INT_2_10_10_10_REV	  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_INT_24_8		  │ 2				       │     2						    │
       │				  │				    N  │					     N	    │
       │				  │				       │						    │
       │				  │				    -  │					     -	    │
       │				  │				    1  │					     1	    │
       │				  │				       │						    │
       │				  │				       │						    │
       │				  │				       │				     ⁢		   │
       │				  │				       │				     c		    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_INT_10F_11F_11F_REV	  │		    --		       │ Special					    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_UNSIGNED_INT_5_9_9_9_REV	  │		    --		       │ Special					    │
       ├──────────────────────────────────┼────────────────────────────────────┼────────────────────────────────────────────────────┤
       │GL_FLOAT_32_UNSIGNED_INT_24_8_REV │		   none		       │		   c (Depth Only)		    │
       └──────────────────────────────────┴────────────────────────────────────┴────────────────────────────────────────────────────┘

       Return values are placed in memory as follows. If format is
       GL_STENCIL_INDEX, GL_DEPTH_COMPONENT, GL_RED, GL_GREEN, or GL_BLUE, a
       single value is returned and the data for the ith pixel in the jth row
       is placed in location j ⁢ width + i.  GL_RGB and GL_BGR return three
       values, GL_RGBA and GL_BGRA return four values for each pixel, with all
       values corresponding to a single pixel occupying contiguous space in
       data. Storage parameters set by glPixelStore(), such as
       GL_PACK_LSB_FIRST and GL_PACK_SWAP_BYTES, affect the way that data is
       written into memory. See glPixelStore() for a description.

NOTES
       Values for pixels that lie outside the window connected to the current
       GL context are undefined.

       If an error is generated, no change is made to the contents of data.

ERRORS
       GL_INVALID_ENUM is generated if format or type is not an accepted
       value.

       GL_INVALID_VALUE is generated if either width or height is negative.

       GL_INVALID_OPERATION is generated if format is GL_STENCIL_INDEX and
       there is no stencil buffer.

       GL_INVALID_OPERATION is generated if format is GL_DEPTH_COMPONENT and
       there is no depth buffer.

       GL_INVALID_OPERATION is generated if format is GL_DEPTH_STENCIL and
       there is no depth buffer or if there is no stencil buffer.

       GL_INVALID_ENUM is generated if format is GL_DEPTH_STENCIL and type is
       not GL_UNSIGNED_INT_24_8 or GL_FLOAT_32_UNSIGNED_INT_24_8_REV.

       GL_INVALID_OPERATION is generated if type is one of
       GL_UNSIGNED_BYTE_3_3_2, GL_UNSIGNED_BYTE_2_3_3_REV,
       GL_UNSIGNED_SHORT_5_6_5, or GL_UNSIGNED_SHORT_5_6_5_REV and format is
       not GL_RGB.

       GL_INVALID_OPERATION is generated if type is one of
       GL_UNSIGNED_SHORT_4_4_4_4, GL_UNSIGNED_SHORT_4_4_4_4_REV,
       GL_UNSIGNED_SHORT_5_5_5_1, GL_UNSIGNED_SHORT_1_5_5_5_REV,
       GL_UNSIGNED_INT_8_8_8_8, GL_UNSIGNED_INT_8_8_8_8_REV,
       GL_UNSIGNED_INT_10_10_10_2, or GL_UNSIGNED_INT_2_10_10_10_REV and
       format is neither GL_RGBA nor GL_BGRA.

       GL_INVALID_OPERATION is generated if a non-zero buffer object name is
       bound to the GL_PIXEL_PACK_BUFFER target and the buffer object's data
       store is currently mapped.

       GL_INVALID_OPERATION is generated if a non-zero buffer object name is
       bound to the GL_PIXEL_PACK_BUFFER target and the data would be packed
       to the buffer object such that the memory writes required would exceed
       the data store size.

       GL_INVALID_OPERATION is generated if a non-zero buffer object name is
       bound to the GL_PIXEL_PACK_BUFFER target and data is not evenly
       divisible into the number of bytes needed to store in memory a datum
       indicated by type.

       GL_INVALID_OPERATION is generated if GL_READ_FRAMEBUFFER_BINDING is
       non-zero, the read framebuffer is complete, and the value of
       GL_SAMPLE_BUFFERS for the read framebuffer is greater than zero.

ASSOCIATED GETS
       glGet() with argument GL_INDEX_MODE

       glGet() with argument GL_PIXEL_PACK_BUFFER_BINDING

SEE ALSO
       glPixelStore(), glReadBuffer()

COPYRIGHT
       Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed
       under the SGI Free Software B License. For details, see
       http://oss.sgi.com/projects/FreeB/.

AUTHORS
       opengl.org

opengl.org			  03/16/2013		      GLREADPIXELS(3G)
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