NAME
	  glCopyPixels - copy pixels in	the frame buffer


     C SPECIFICATION
	  void glCopyPixels( GLint x,
			     GLint y,
			     GLsizei width,
			     GLsizei height,
			     GLenum type )


     PARAMETERS
	  x, y Specify the window coordinates of the lower left	corner
	       of the rectangular region of pixels to be copied.

	  width, height
	       Specify the dimensions of the rectangular region	of
	       pixels to be copied.  Both must be nonnegative.

	  type Specifies whether color values, depth values, or
	       stencil values are to be	copied.	 Symbolic constants
	       GL_COLOR, GL_DEPTH, and GL_STENCIL are accepted.

     DESCRIPTION
	  glCopyPixels copies a	screen-aligned rectangle of pixels
	  from the specified frame buffer location to a	region
	  relative to the current raster position.  Its	operation is
	  well defined only if the entire pixel	source region is
	  within the exposed portion of	the window.  Results of	copies
	  from outside the window, or from regions of the window that
	  are not exposed, are hardware	dependent and undefined.

	  x and	y specify the window coordinates of the	lower left
	  corner of the	rectangular region to be copied.  width	and
	  height specify the dimensions	of the rectangular region to
	  be copied.  Both width and height must not be	negative.

	  Several parameters control the processing of the pixel data
	  while	it is being copied.  These parameters are set with
	  three	commands:  glPixelTransfer, glPixelMap,	and
	  glPixelZoom.	This reference page describes the effects on
	  glCopyPixels of most,	but not	all, of	the parameters
	  specified by these three commands.

	  glCopyPixels copies values from each pixel with the lower
	  left-hand 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 copied in	row order from the lowest to
	  the highest row, left	to right in each row.

	  type specifies whether color,	depth, or stencil data is to
	  be copied.  The details of the transfer for each data	type
	  are as follows:

	  GL_COLOR	 Indices or RGBA colors	are read from the
			 buffer	currently specified as the read	source
			 buffer	(see glReadBuffer).  If	the GL is in
			 color index mode, each	index that is read
			 from this buffer is converted to a fixed-
			 point format with an unspecified number of
			 bits to the right of the binary point.	 Each
			 index is then shifted left by GL_INDEX_SHIFT
			 bits, and added to GL_INDEX_OFFSET.  If
			 GL_INDEX_SHIFT	is negative, the shift is to
			 the right.  In	either case, zero bits fill
			 otherwise unspecified bit locations in	the
			 result.  If GL_MAP_COLOR is true, the index
			 is replaced with the value that it references
			 in lookup table GL_PIXEL_MAP_I_TO_I.  Whether
			 the lookup replacement	of the index is	done
			 or not, the integer part of the index is then
			 ANDed with 2b-1, where	b is the number	of
			 bits in a color index buffer.

			 If the	GL is in RGBA mode, the	red, green,
			 blue, and alpha components of each pixel that
			 is read are converted to an internal
			 floating-point	format with unspecified
			 precision.  The conversion maps the largest
			 representable component value to 1.0, and
			 component value 0 to 0.0.  The	resulting
			 floating-point	color values are then
			 multiplied by GL_c_SCALE and added to
			 GL_c_BIAS, where c is RED, GREEN, BLUE, and
			 ALPHA for the respective color	components.
			 The results are clamped to the	range [0,1].
			 If GL_MAP_COLOR is true, each color component
			 is scaled by the size of lookup table
			 GL_PIXEL_MAP_c_TO_c, then replaced by the
			 value that it references in that table.  c is
			 R, G, B, or A.

			 The GL	then converts the resulting indices or
			 RGBA colors to	fragments by attaching the
			 current raster	position z coordinate and
			 texture coordinates to	each pixel, then
			 assigning window coordinates (x +i,y +j),
			 where (x ,y ) is the current raster position,
			 and the pixel was the ith pixel in the	jth
			 row.  These pixel fragments are then treated
			 just like the fragments generated by
			 rasterizing points, lines, or polygons.
			 Texture mapping, fog, and all the fragment
			 operations are	applied	before the fragments
			 are written to	the frame buffer.

	  GL_DEPTH	 Depth values are read from the	depth buffer
			 and converted directly	to an internal
			 floating-point	format with unspecified
			 precision.  The resulting floating-point
			 depth value is	then multiplied	by
			 GL_DEPTH_SCALE	and added to GL_DEPTH_BIAS.
			 The result is clamped to the range [0,1].

			 The GL	then converts the resulting depth
			 components to fragments by attaching the
			 current raster	position color or color	index
			 and texture coordinates to each pixel,	then
			 assigning window coordinates (x +i,y +j),
			 where (x ,y ) is the current raster position,
			 and the pixel was the ith pixel in the	jth
			 row.  These pixel fragments are then treated
			 just like the fragments generated by
			 rasterizing points, lines, or polygons.
			 Texture mapping, fog, and all the fragment
			 operations are	applied	before the fragments
			 are written to	the frame buffer.

	  GL_STENCIL	 Stencil indices are read from the stencil
			 buffer	and converted to an internal fixed-
			 point format with an unspecified number of
			 bits to the right of the binary point.	 Each
			 fixed-point index is then shifted left	by
			 GL_INDEX_SHIFT	bits, and added	to
			 GL_INDEX_OFFSET.  If GL_INDEX_SHIFT is
			 negative, the shift is	to the right.  In
			 either	case, zero bits	fill otherwise
			 unspecified bit locations in the result.  If
			 GL_MAP_STENCIL	is true, the index is replaced
			 with the value	that it	references in lookup
			 table GL_PIXEL_MAP_S_TO_S.  Whether the
			 lookup	replacement of the index is done or
			 not, the integer part of the index is then
			 ANDed with 2b-1, where	b is the number	of
			 bits in the stencil buffer.  The resulting
			 stencil indices are then written to the
			 stencil buffer	such that the index read from
			 the ith location of the jth row is written to
			 location (x +i,y +j), where (x	,y ) is	the
			 current raster	position.  Onlyrthe pixel
			 ownership test, the scissor test, and the
			 stencil writemask affect these	write
			 operations.

	  The rasterization described thus far assumes pixel zoom
	  factors of 1.0.  If
	  glPixelZoom is used to change	the x and y pixel zoom
	  factors, pixels are converted	to fragments as	follows.  If
	  (x , y ) is the current raster position, and a given pixel
	  isrin	the ith	location in the	jth row	of the source pixel
	  rectangle, then fragments are	generated for pixels whose
	  centers are in the rectangle with corners at

			     (x	+zoom i, y +zoom j)
			       r     x	  r	y
				      and
			 (x +zoom (i+1), y +zoom (j+1))
			   r	 x	  r	y
	  where	zoom  is the value of GL_ZOOM_X	and zoom  is the value
	  of GL_ZOOM_Y.					y

     EXAMPLES
	  To copy the color pixel in the lower left corner of the
	  window to the	current	raster position, use glCopyPixels(0,
	  0, 1,	1, GL_COLOR);

     NOTES
	  Modes	specified by glPixelStore have no effect on the
	  operation of glCopyPixels.

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

	  GL_INVALID_VALUE is generated	if either width	or height is
	  negative.

	  GL_INVALID_OPERATION is generated if type is GL_DEPTH	and
	  there	is no depth buffer.

	  GL_INVALID_OPERATION is generated if type is GL_STENCIL and
	  there	is no stencil buffer.

	  GL_INVALID_OPERATION is generated if glCopyPixels is
	  executed between the execution of glBegin and	the
	  corresponding	execution of glEnd.

     ASSOCIATED	GETS
	  glGet	with argument GL_CURRENT_RASTER_POSITION
	  glGet	with argument GL_CURRENT_RASTER_POSITION_VALID

     SEE ALSO
	  glDepthFunc, glDrawBuffer, glDrawPixels, glPixelMap,
	  glPixelTransfer, glPixelZoom,	glRasterPos, glReadBuffer,
	  glReadPixels,	glStencilFunc