drawpixels.html

计算机图形学～想必是很多人需要的～在此共享一下

	       If <STRONG>GL_MAP_COLOR</STRONG> is true,	each color component is	scaled
	       by the size of lookup table <STRONG>GL_PIXEL_MAP_c_TO_c</STRONG>,	then
	       replaced	by the value that it references	in that	table.
	       <EM>c</EM> is R, G, B, or	A respectively.

	       The GL then converts the	resulting RGBA colors to
	       fragments by attaching the current raster position <EM>z</EM>
	       coordinate and texture coordinates to each pixel, then
	       assigning x and y window	coordinates to the nth
	       fragment	such that

			       x  = x  + n mod width
				n    r
				y  = y	+ | n/width |
				 n    r

	       where (x	,y ) is	the current raster position.  These
	       pixel fr<STRONG>a</STRONG>gm<STRONG>e</STRONG>nts 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.

	  <STRONG>GL_RED</STRONG>
	       Each pixel is a single red component.  This component
	       is converted to the internal floating-point format in
	       the same	way the	red component of an RGBA pixel is. It
	       is then converted to an RGBA pixel with green and blue
	       set to 0, and alpha set to 1.  After this conversion,
	       the pixel is treated as if it had been read as an RGBA
	       pixel.

	  <STRONG>GL_GREEN</STRONG>
	       Each pixel is a single green component.	This component
	       is converted to the internal floating-point format in
	       the same	way the	green component	of an RGBA pixel is.
	       It is then converted to an RGBA pixel with red and blue
	       set to 0, and alpha set to 1.  After this conversion,
	       the pixel is treated as if it had been read as an RGBA
	       pixel.

	  <STRONG>GL_BLUE</STRONG>
	       Each pixel is a single blue component.  This component
	       is converted to the internal floating-point format in
	       the same	way the	blue component of an RGBA pixel	is.
	       It is then converted to an RGBA pixel with red and
	       green set to 0, and alpha set to	1.  After this
	       conversion, the pixel is	treated	as if it had been read
	       as an RGBA pixel.

	  <STRONG>GL_ALPHA</STRONG>
	       Each pixel is a single alpha component.	This component
	       is converted to the internal floating-point format in
	       the same	way the	alpha component	of an RGBA pixel is.
	       It is then converted to an RGBA pixel with red, green,
	       and blue	set to 0.  After this conversion, the pixel is
	       treated as if it	had been read as an RGBA pixel.

	  <STRONG>GL_RGB</STRONG>
	       Each pixel is a three-component group:  red first,
	       followed	by green, followed by blue.  Each component is
	       converted to the	internal floating-point	format in the
	       same way	the red, green,	and blue components of an RGBA
	       pixel are.  The color triple is converted to an RGBA
	       pixel with alpha	set to 1.  After this conversion, the
	       pixel is	treated	as if it had been read as an RGBA
	       pixel.

	  <STRONG>GL_LUMINANCE</STRONG>
	       Each pixel is a single luminance	component.  This
	       component is converted to the internal floating-point
	       format in the same way the red component	of an RGBA
	       pixel is.  It is	then converted to an RGBA pixel	with
	       red, green, and blue set	to the converted luminance
	       value, and alpha	set to 1.  After this conversion, the
	       pixel is	treated	as if it had been read as an RGBA
	       pixel.

	  <STRONG>GL_LUMINANCE_ALPHA</STRONG>
	       Each pixel is a two-component group:  luminance first,
	       followed	by alpha.  The two components are converted to
	       the internal floating-point format in the same way the
	       red component of	an RGBA	pixel is.  They	are then
	       converted to an RGBA pixel with red, green, and blue
	       set to the converted luminance value, and alpha set to
	       the converted alpha value.  After this conversion, the
	       pixel is	treated	as if it had been read as an RGBA
	       pixel.

	  The following	table summarizes the meaning of	the valid
	  constants for	the <EM>type</EM> parameter:











	  ____________________________________________________________
	  |	 <EM>type</EM>	     |		 <EM>corresponding</EM> <EM>type</EM>	      |
	  <EM>|</EM>__________________<EM>|</EM>________________________________________|
	  |GL_UNSIGNED_BYTE  |	       unsigned	8-bit integer	      |
	  |	GL_BYTE	     |		signed 8-bit integer	      |
	  |    GL_BITMAP     | single bits in unsigned 8-bit integers |
	  |GL_UNSIGNED_SHORT |	      unsigned 16-bit integer	      |
	  |    GL_SHORT	     |	       signed 16-bit integer	      |
	  | GL_UNSIGNED_INT  |	      unsigned 32-bit integer	      |
	  |	GL_INT	     |		   32-bit integer	      |
	  |    GL_FLOAT	     |	  single-precision floating-point     |
	  <EM>|</EM>__________________<EM>|</EM>________________________________________|


	  The rasterization described so far assumes pixel zoom
	  factors of 1.	 If
	  <STRONG>glPixelZoom</STRONG> 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	<STRONG>t</STRONG>he nth	column and mth row of the pixel	rectangle,
	  then fragments are generated for pixels whose	centers	are in
	  the rectangle	with corners at

			       (x +zoom	n, y +zoom m)
				 r     x    r	  y
			   (x +zoom (n+1), y +zoom (m+1))
			     r	   x	    r	  y

	  where	zoom  is the value of <STRONG>GL_ZOOM_X</STRONG>	and zoom  is the value
	  of <STRONG>GL_ZOOM_Y</STRONG>.					y

     <STRONG>ERRORS</STRONG>
	  <STRONG>GL_INVALID_VALUE</STRONG> is generated	if either <EM>width</EM>	or <EM>height</EM> is
	  negative.

	  <STRONG>GL_INVALID_ENUM</STRONG> is generated if <EM>format</EM> or <EM>type</EM> is not	one of
	  the accepted values.

	  <STRONG>GL_INVALID_OPERATION</STRONG> is generated 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 the GL is in color	index mode.

	  <STRONG>GL_INVALID_ENUM</STRONG> is generated if <EM>type</EM> is <STRONG>GL_BITMAP</STRONG> and	<EM>format</EM>
	  is not either	<STRONG>GL_COLOR_INDEX</STRONG> or <STRONG>GL_STENCIL_INDEX</STRONG>.

	  <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 <STRONG>glDrawPixels</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_CURRENT_RASTER_POSITION</STRONG>
	  <STRONG>glGet</STRONG>	with argument <STRONG>GL_CURRENT_RASTER_POSITION_VALID</STRONG>

     <STRONG>SEE</STRONG> <STRONG>ALSO</STRONG>
	  <STRONG>glAlphaFunc</STRONG>, <STRONG>glBlendFunc</STRONG>, <STRONG>glCopyPixels</STRONG>, <STRONG>glDepthFunc</STRONG>,
	  <STRONG>glLogicOp</STRONG>, <STRONG>glPixelMap</STRONG>, <STRONG>glPixelStore</STRONG>, <STRONG>glPixelTransfer</STRONG>,
	  <STRONG>glPixelZoom</STRONG>, <STRONG>glRasterPos</STRONG>, <STRONG>glReadPixels</STRONG>, <STRONG>glScissor</STRONG>,
	  <STRONG>glStencilFunc</STRONG>











































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