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 *pixels )


     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_COLOR_INDEX,
	       GL_STENCIL_INDEX, GL_DEPTH_COMPONENT, GL_RED, GL_GREEN,
	       GL_BLUE,	GL_ALPHA, GL_RGB, GL_RGBA, GL_LUMINANCE, and
	       GL_LUMINANCE_ALPHA.

	  type Specifies the data type of the pixel data.  Must	be one
	       of GL_UNSIGNED_BYTE, GL_BYTE, GL_BITMAP,
	       GL_UNSIGNED_SHORT, GL_SHORT, GL_UNSIGNED_INT, GL_INT,
	       or GL_FLOAT.

	  pixels
	       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
	  pixels.  Several parameters control the processing of	the
	  pixel	data before it is placed into client memory.  These
	  parameters are set with three	commands:  glPixelStore,
	  glPixelTransfer, and glPixelMap.  This reference page
	  describes the	effects	on glReadPixels	of most, but not all
	  of the parameters specified by these three commands.

	  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_COLOR_INDEX
		    Color indices are read from	the color buffer
		    selected by	glReadBuffer.  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_COLOR is	GL_TRUE,
		    indices are	replaced by their mappings in the
		    table GL_PIXEL_MAP_I_TO_I.

	  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_RED

	  GL_GREEN

	  GL_BLUE

	  GL_ALPHA

	  GL_RGB

	  GL_RGBA

	  GL_LUMINANCE

	  GL_LUMINANCE_ALPHA
		    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
		    glReadBuffer.  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. Indices are then replaced by the
		    red, green,	blue, and alpha	values obtained	by
		    indexing the tables	GL_PIXEL_MAP_I_TO_R,
		    GL_PIXEL_MAP_I_TO_G, GL_PIXEL_MAP_I_TO_B, and
		    GL_PIXEL_MAP_I_TO_A.  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	glReadBuffer.  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
		    GL_c_SCALE and added to GL_c_BIAS, where c is RED,
		    GREEN, BLUE, or ALPHA. Finally, if GL_MAP_COLOR is
		    GL_TRUE, 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 GL_PIXEL_MAP_c_TO_c, where c is R, G,	B, or
		    A.

		    Unneeded data is then discarded.  For example,
		    GL_RED discards the	green, blue, and alpha
		    components,	while GL_RGB discards only the alpha
		    component.	GL_LUMINANCE computes a	single-
		    component value as the sum of the red, green, and
		    blue components, and GL_LUMINANCE_ALPHA 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
	  glPixelTransfer.  The	lookup table contents themselves are
	  specified by glPixelMap.

	  Finally, the indices or components are converted to the
	  proper format, as specified by type.	If format is
	  GL_COLOR_INDEX or 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_ALPHA, GL_RGB,
	  GL_RGBA, GL_LUMINANCE, or GL_LUMINANCE_ALPHA 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 mask | component	conversion |
	    |__________________|____________|______________________|
	    |		       |	    |			   |
	    |GL_UNSIGNED_BYTE  |   2^8-1    |	    (2^8-1)c	   |
	    |	  GL_BYTE      |   2^7-1    |	 [(2^8-1)c-1]/2	   |
	    |	 GL_BITMAP     |     1	    |	       1	   |
	    |GL_UNSIGNED_SHORT |   2^16-1   |	   (2^16-1)c	   |
	    |	 GL_SHORT      |   2^15-1   |	[(2^16-1)c-1]/2	   |
	    | GL_UNSIGNED_INT  |   2^32-1   |	   (2^32-1)c	   |
	    |	  GL_INT       |   2^31-1   |	[(2^32-1)c-1]/2	   |
	    |	 GL_FLOAT      |    none    |	       c	   |
	    |__________________|____________|______________________|

	  Return values	are placed in memory as	follows.  If format is
	  GL_COLOR_INDEX, GL_STENCIL_INDEX, GL_DEPTH_COMPONENT,
	  GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, or GL_LUMINANCE,	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
	  returns three	values,	GL_RGBA	returns	four values, and
	  GL_LUMINANCE_ALPHA returns two values	for each pixel,	with
	  all values corresponding to a	single pixel occupying
	  contiguous space in pixels.  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 pixels.

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

	  GL_INVALID_ENUM is generated if type is GL_BITMAP and	format
	  is not GL_COLOR_INDEX	or GL_STENCIL_INDEX.

	  GL_INVALID_VALUE is generated	if either width	or height is
	  negative.

	  GL_INVALID_OPERATION is generated if format is
	  GL_COLOR_INDEX and the color buffers store RGBA color
	  components.

	  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 glReadPixels is
	  executed between the execution of glBegin and	the
	  corresponding	execution of glEnd.

     ASSOCIATED	GETS
	  glGet	with argument GL_INDEX_MODE

     SEE ALSO
	  glCopyPixels,	glDrawPixels, glPixelMap, glPixelStore,
	  glPixelTransfer,
	  glReadBuffer