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<HTML><BODY><PRE>     <STRONG>NAME</STRONG>	  <STRONG>glReadPixels</STRONG> - read a	block of pixels	from the frame buffer     <STRONG>C</STRONG> <STRONG>SPECIFICATION</STRONG>	  void <STRONG>glReadPixels</STRONG>( GLint <EM>x</EM>,			     GLint <EM>y</EM>,			     GLsizei <EM>width</EM>,			     GLsizei <EM>height</EM>,			     GLenum <EM>format</EM>,			     GLenum <EM>type</EM>,			     GLvoid *<EM>pixels</EM> )     <STRONG>PARAMETERS</STRONG>	  <EM>x</EM>, <EM>y</EM> 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.	  <EM>width</EM>, <EM>height</EM>	       Specify the dimensions of the pixel rectangle.  <EM>width</EM>	       and <EM>height</EM> of one correspond to a single	pixel.	  <EM>format</EM>	       Specifies the format of the pixel data.	The following	       symbolic	values are accepted:  <STRONG>GL_COLOR_INDEX</STRONG>,	       <STRONG>GL_STENCIL_INDEX</STRONG>, <STRONG>GL_DEPTH_COMPONENT</STRONG>, <STRONG>GL_RED</STRONG>, <STRONG>GL_GREEN</STRONG>,	       <STRONG>GL_BLUE</STRONG>,	<STRONG>GL_ALPHA</STRONG>, <STRONG>GL_RGB</STRONG>, <STRONG>GL_RGBA</STRONG>, <STRONG>GL_LUMINANCE</STRONG>, and	       <STRONG>GL_LUMINANCE_ALPHA</STRONG>.	  <EM>type</EM> Specifies the data type of the pixel data.  Must	be one	       of <STRONG>GL_UNSIGNED_BYTE</STRONG>, <STRONG>GL_BYTE</STRONG>, <STRONG>GL_BITMAP</STRONG>,	       <STRONG>GL_UNSIGNED_SHORT</STRONG>, <STRONG>GL_SHORT</STRONG>, <STRONG>GL_UNSIGNED_INT</STRONG>, <STRONG>GL_INT</STRONG>,	       or <STRONG>GL_FLOAT</STRONG>.	  <EM>pixels</EM>	       Returns the pixel data.     <STRONG>DESCRIPTION</STRONG>	  <STRONG>glReadPixels</STRONG> returns pixel data from the frame buffer,	  starting with	the pixel whose	lower left corner is at	  location (<EM>x</EM>, <EM>y</EM>), into	client memory starting at location	  <EM>pixels</EM>.  Several parameters control the processing of	the	  pixel	data before it is placed into client memory.  These	  parameters are set with three	commands:  <STRONG>glPixelStore</STRONG>,	  <STRONG>glPixelTransfer</STRONG>, and <STRONG>glPixelMap</STRONG>.  This reference page	  describes the	effects	on <STRONG>glReadPixels</STRONG>	of most, but not all	  of the parameters specified by these three commands.	  <STRONG>glReadPixels</STRONG> returns values from each	pixel with lower left	  corner at (<EM>x</EM> + i, <EM>y</EM> +	j) for 0<EM>&lt;</EM>i&lt;<EM>width</EM>  and 0<EM>&lt;</EM>j&lt;<EM>height</EM>.	  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.	  <EM>format</EM> specifies the format for the returned pixel values;	  accepted values are:	  <STRONG>GL_COLOR_INDEX</STRONG>		    Color indices are read from	the color buffer		    selected by	<STRONG>glReadBuffer</STRONG>.  Each index is converted		    to fixed point, shifted left or right depending on		    the	value and sign of <STRONG>GL_INDEX_SHIFT</STRONG>, and added to		    <STRONG>GL_INDEX_OFFSET</STRONG>. If	<STRONG>GL_MAP_COLOR</STRONG> is	<STRONG>GL_TRUE</STRONG>,		    indices are	replaced by their mappings in the		    table <STRONG>GL_PIXEL_MAP_I_TO_I</STRONG>.	  <STRONG>GL_STENCIL_INDEX</STRONG>		    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		    <STRONG>GL_INDEX_SHIFT</STRONG>, and	added to <STRONG>GL_INDEX_OFFSET</STRONG>. If		    <STRONG>GL_MAP_STENCIL</STRONG> is <STRONG>GL_TRUE</STRONG>, indices are replaced by		    their mappings in the table	<STRONG>GL_PIXEL_MAP_S_TO_S</STRONG>.	  <STRONG>GL_DEPTH_COMPONENT</STRONG>		    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 <STRONG>GL_DEPTH_SCALE</STRONG>, added	to		    <STRONG>GL_DEPTH_BIAS</STRONG>, and finally clamped to the range		    [0,1].	  <STRONG>GL_RED</STRONG>	  <STRONG>GL_GREEN</STRONG>	  <STRONG>GL_BLUE</STRONG>	  <STRONG>GL_ALPHA</STRONG>	  <STRONG>GL_RGB</STRONG>	  <STRONG>GL_RGBA</STRONG>	  <STRONG>GL_LUMINANCE</STRONG>	  <STRONG>GL_LUMINANCE_ALPHA</STRONG>		    Processing differs depending on whether color		    buffers store color	indices	or RGBA	color		    components.	 If color indices are stored, they are		    read from the color	buffer selected	by		    <STRONG>glReadBuffer</STRONG>.  Each	index is converted to fixed		    point, shifted left	or right depending on the		    value and sign of <STRONG>GL_INDEX_SHIFT</STRONG>, and added	to		    <STRONG>GL_INDEX_OFFSET</STRONG>. Indices are then replaced by the		    red, green,	blue, and alpha	values obtained	by		    indexing the tables	<STRONG>GL_PIXEL_MAP_I_TO_R</STRONG>,		    <STRONG>GL_PIXEL_MAP_I_TO_G</STRONG>, <STRONG>GL_PIXEL_MAP_I_TO_B</STRONG>, and		    <STRONG>GL_PIXEL_MAP_I_TO_A</STRONG>.  Each table must be of	size		    2^n, but n may be different	for different tables.		    Before an index is used to look up a value in a		    table of size 2^n, it must be masked against 2^n-		    1.		    If RGBA color components are stored	in the color		    buffers, they are read from	the color buffer		    selected by	<STRONG>glReadBuffer</STRONG>.  Each color component is		    converted to floating point	such that zero		    intensity maps to 0.0 and full intensity maps to		    1.0.  Each component is then multiplied by		    <STRONG>GL_c_SCALE</STRONG> and added to <STRONG>GL_c_BIAS</STRONG>, where <EM>c</EM> is RED,		    GREEN, BLUE, or ALPHA. Finally, if <STRONG>GL_MAP_COLOR</STRONG> is		    <STRONG>GL_TRUE</STRONG>, each component is clamped to the range		    [0,1], scaled to the size of its corresponding		    table, and is then replaced	by its mapping in the		    table <STRONG>GL_PIXEL_MAP_c_TO_c</STRONG>, where <EM>c</EM> is R, G,	B, or		    A.		    Unneeded data is then discarded.  For example,		    <STRONG>GL_RED</STRONG> discards the	green, blue, and alpha		    components,	while <STRONG>GL_RGB</STRONG> discards only the alpha		    component.	<STRONG>GL_LUMINANCE</STRONG> computes a	single-		    component value as the sum of the red, green, and		    blue components, and <STRONG>GL_LUMINANCE_ALPHA</STRONG> does the		    same, while	keeping	alpha as a second value.  The		    final values are clamped to	the range [0,1].	  The shift, scale, bias, and lookup factors just described	  are all specified by	  <STRONG>glPixelTransfer</STRONG>.  The	lookup table contents themselves are	  specified by <STRONG>glPixelMap</STRONG>.	  Finally, the indices or components are converted to the	  proper format, as specified by <EM>type</EM>.	If <EM>format</EM> is	  <STRONG>GL_COLOR_INDEX</STRONG> or <STRONG>GL_STENCIL_INDEX</STRONG> and <EM>type</EM> is not <STRONG>GL_FLOAT</STRONG>,	  each index is	masked with the	mask value given in the	  following table.  If <EM>type</EM> is <STRONG>GL_FLOAT</STRONG>, then each integer	  index	is converted to	single-precision floating-point	  format.	  If <EM>format</EM> is <STRONG>GL_RED</STRONG>, <STRONG>GL_GREEN</STRONG>, <STRONG>GL_BLUE</STRONG>, <STRONG>GL_ALPHA</STRONG>, <STRONG>GL_RGB</STRONG>,	  <STRONG>GL_RGBA</STRONG>, <STRONG>GL_LUMINANCE</STRONG>, or <STRONG>GL_LUMINANCE_ALPHA</STRONG> and <EM>type</EM>	is not	  <STRONG>GL_FLOAT</STRONG>, each component is multiplied by the	multiplier	  shown	in the following table.	 If type is <STRONG>GL_FLOAT</STRONG>, 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).	    _______________________________________________________	    |	   <EM>type</EM>	       | <EM>index</EM> <EM>mask</EM> | <EM>component</EM>	<EM>conversion</EM> |	    <EM>|</EM>__________________<EM>|</EM>____________<EM>|</EM>______________________|	    |		       |	    |			   |	    |<STRONG>GL_UNSIGNED_BYTE</STRONG>  |   2^8-1    |	    (2^8-1)c	   |	    |	  <STRONG>GL_BYTE</STRONG>      |   2^7-1    |	 [(2^8-1)c-1]/2	   |	    |	 <STRONG>GL_BITMAP</STRONG>     |     1	    |	       1	   |	    |<STRONG>GL_UNSIGNED_SHORT</STRONG> |   2^16-1   |	   (2^16-1)c	   |	    |	 <STRONG>GL_SHORT</STRONG>      |   2^15-1   |	[(2^16-1)c-1]/2	   |	    | <STRONG>GL_UNSIGNED_INT</STRONG>  |   2^32-1   |	   (2^32-1)c	   |	    |	  <STRONG>GL_INT</STRONG>       |   2^31-1   |	[(2^32-1)c-1]/2	   |	    |	 <STRONG>GL_FLOAT</STRONG>      |    none    |	       c	   |	    <EM>|</EM>__________________<EM>|</EM>____________<EM>|</EM>______________________|	  Return values	are placed in memory as	follows.  If <EM>format</EM> is	  <STRONG>GL_COLOR_INDEX</STRONG>, <STRONG>GL_STENCIL_INDEX</STRONG>, <STRONG>GL_DEPTH_COMPONENT</STRONG>,	  <STRONG>GL_RED</STRONG>, <STRONG>GL_GREEN</STRONG>, <STRONG>GL_BLUE</STRONG>, <STRONG>GL_ALPHA</STRONG>, or <STRONG>GL_LUMINANCE</STRONG>,	a	  single value is returned and the data	for the	ith pixel in	  the jth row is placed	in location (j)	width +	i.  <STRONG>GL_RGB</STRONG>	  returns three	values,	<STRONG>GL_RGBA</STRONG>	returns	four values, and	  <STRONG>GL_LUMINANCE_ALPHA</STRONG> returns two values	for each pixel,	with	  all values corresponding to a	single pixel occupying	  contiguous space in <EM>pixels</EM>.  Storage parameters set by	  <STRONG>glPixelStore</STRONG>,	such as	<STRONG>GL_PACK_LSB_FIRST</STRONG> and	  <STRONG>GL_PACK_SWAP_BYTES</STRONG>, affect the way that data is written into	  memory.  See <STRONG>glPixelStore</STRONG> for	a description.     <STRONG>NOTES</STRONG>	  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 <EM>pixels</EM>.     <STRONG>ERRORS</STRONG>	  <STRONG>GL_INVALID_ENUM</STRONG> is generated if <EM>format</EM> or <EM>type</EM> is not	an	  accepted value.	  <STRONG>GL_INVALID_ENUM</STRONG> is generated if <EM>type</EM> is <STRONG>GL_BITMAP</STRONG> and	<EM>format</EM>	  is not <STRONG>GL_COLOR_INDEX</STRONG>	or <STRONG>GL_STENCIL_INDEX</STRONG>.	  <STRONG>GL_INVALID_VALUE</STRONG> is generated	if either <EM>width</EM>	or <EM>height</EM> is	  negative.	  <STRONG>GL_INVALID_OPERATION</STRONG> is generated if <EM>format</EM> is	  <STRONG>GL_COLOR_INDEX</STRONG> and the color buffers store RGBA color	  components.	  <STRONG>GL_INVALID_OPERATION</STRONG> is generated if <EM>format</EM> is	  <STRONG>GL_STENCIL_INDEX</STRONG> and there is	no stencil buffer.	  <STRONG>GL_INVALID_OPERATION</STRONG> is generated if <EM>format</EM> is	  <STRONG>GL_DEPTH_COMPONENT</STRONG> and there is no depth buffer.	  <STRONG>GL_INVALID_OPERATION</STRONG> is generated if <STRONG>glReadPixels</STRONG> is	  executed between the execution of <STRONG>glBegin</STRONG> and	the	  corresponding	execution of <STRONG>glEnd</STRONG>.     <STRONG>ASSOCIATED</STRONG>	<STRONG>GETS</STRONG>	  <STRONG>glGet</STRONG>	with argument <STRONG>GL_INDEX_MODE</STRONG>     <STRONG>SEE</STRONG> <STRONG>ALSO</STRONG>	  <STRONG>glCopyPixels</STRONG>,	<STRONG>glDrawPixels</STRONG>, <STRONG>glPixelMap</STRONG>, <STRONG>glPixelStore</STRONG>,	  <STRONG>glPixelTransfer</STRONG>,	  <STRONG>glReadBuffer</STRONG></PRE></BODY></HTML>
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