glPixelTexGenSGIX(3G)	       OpenGL Reference		 glPixelTexGenSGIX(3G)



NAME
     glPixelTexGenSGIX - control the generation	of texture coordinates from
     pixel groups


C SPECIFICATION
     void glPixelTexGenSGIX( GLenum mode )


PARAMETERS
     mode  Specifies a mode of generating texture coordinates from pixel
	   groups, and corresponds to the number of components in the pixel
	   group.  Must	be one of the following:  GL_NONE, GL_RGBA, GL_RGB,
	   GL_LUMINANCE_ALPHA, GL_LUMINANCE.
	   GL_PIXEL_TEX_GEN_ALPHA_REPLACE_SGIX,
	   GL_PIXEL_TEX_GEN_ALPHA_NO_REPLACE_SGIX,
	   GL_PIXEL_TEX_GEN_ALPHA_MS_SGIX, or GL_PIXEL_TEX_GEN_ALPHA_LS_SGIX.
	   The default value is	GL_NONE.


DESCRIPTION
     glPixelTexGenSGIX affects the result of the glDrawPixels and glCopyPixels
     commands.	It allows the color components of pixel	groups to be used as
     texture coordinates, effectively converting a color image into a texture
     coordinate	image.	Because	texture	mapping	is essentially a
     multidimensional table lookup, this conversion supports multidimensional
     color lookups for images.	Such multidimensional lookups can be used to
     implement very accurate color space conversions.

     If	the parameter is GL_LUMINANCE, there must be a valid 1D	texture
     enabled, which can	have any internal format. The external format of the
     pixel group must be GL_LUMINANCE; the luminance component will be used to
     generate an s texture coordinate.

     If	the parameter is GL_LUMINANCE_ALPHA, there must	be a valid 2D texture
     enabled, which can	have any internal format. The external format of the
     pixel group must be GL_LUMINANCE_ALPHA; the luminance and alpha
     components	will be	used to	generate s and t texture coordinates,
     respectively.

     If	the parameter is GL_RGB, there must be a valid 3D texture enabled,
     which can have any	internal format. The external format of	the pixel
     group must	be GL_RGB; the red, green, and blue components will be used to
     generate s, t and r texture coordinates, respectively.

     If	the parameter is GL_RGBA, there	must be	a valid	4D texture enabled,
     which can have any	internal format. The external format of	the pixel
     group must	be GL_RGBA; the	red, green, blue, and alpha components will be
     used to generate s, t, r, and q texture coordinates, respectively.






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glPixelTexGenSGIX(3G)	       OpenGL Reference		 glPixelTexGenSGIX(3G)



     The texture components generated in this way are normalized to the	range
     [0..1] based on the external type of the pixel group.

     The result	of the texture-mapping operation is texture-blended onto a
     fragment which has	the attributes of the current raster position.
     Therefore the desired texture environment mode must be set, based on the
     internal format of	the texture. For example, it is	possible to have a
     pixel group with the format GL_RGBA generate texture coordinates into a
     4D	texture; yet the internal format of the	texture	can be GL_LUMINANCE
     which is undefined	for GL_DECAL mode in the texture environment.

     If	the parameter is GL_NONE, pixel	texture	is disabled, the default.

     A useful mode of pixel texture is to have a pixel group with the external
     format GL_RGBA index into a 3D texture, and have the alpha	component
     combined with the texel output. In	this case the internal format of the
     texture must have alpha, and that alpha can be replaced by	the "incoming"
     alpha by calling glPixelTexGenSGIX	with the parameter
     GL_PIXEL_TEX_GEN_ALPHA_REPLACE_SGIX. Since	the incoming pixel group has
     four components, the texture must be a 4D texture;	since the alpha	is not
     intended for lookup, the 4D texture can have a q size of one which	is
     essentially a 3D texture. The "alpha replace" mode	can be turned off by
     calling glPixelTexGenSGIX with the	parameter
     GL_PIXEL_TEX_GEN_ALPHA_NO_REPLACE_SGIX, the default.

     Currently Silicon Graphics	machines that support glPixelTexGenSGIX	in
     hardware do not have quad-linear interpolation. So	the fourth dimension
     of	interpolation, q, defaults to nearest-neighbor mode (the other
     dimensions	may still be linear, depending on the glTexParameter call).

     In	order to achieve a true	quad-linearly interpolated result, a two-pass
     operation is required, which uses the frame buffer	blend for the last
     interpolation. For	this purpose special ceiling/floor functions for the q
     dimension have been supplied, which can be	referenced in the man pages
     for glTexParameter. These are

     GL_PIXEL_TEX_GEN_Q_CEILING_SGIX

     GL_PIXEL_TEX_GEN_Q_FLOOR_SGIX

     GL_PIXEL_TEX_GEN_Q_ROUND_SGIX

     On	the second pass, use the "alpha	replace" mode (described above)	to
     combine the incoming alpha	with the output	(as well as using it in	the
     fourth dimensional	texture	lookup). The original alpha, however, is not
     usable as a fractional interpolation value	until it gets left-shifted by
     the number	of bits	corresponding to the logarithm (base 2)	of the q size
     of	4D texture. The	alpha can be effectively left-shifted using the
     texture color table. Since	the table has limited precision, a mode	is
     supplied which will left-shift the	alpha by 4 bits	before combining it
     with the output. To use it	call glPixelTexGenSGIX with the	parameter
     GL_PIXEL_TEX_GEN_ALPHA_LS_SGIX. The default, with no left shift, is



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glPixelTexGenSGIX(3G)	       OpenGL Reference		 glPixelTexGenSGIX(3G)



     GL_PIXEL_TEX_GEN_ALPHA_MS_SGIX (MS	and LS refer to	"most significant" and
     "least significant"). This	produces excellent results for 8 and 12	bit
     components, for different texture q sizes.



ERRORS
     GL_INVALID_ENUM is	generated when mode is not an accepted defined value.

MACHINE	DEPENDENCIES
     glPixelTexGenSGIX is supported only on High Impact	and Maximum Impact
     systems; however, the 1D and 2D modes (GL_LUMINANCE and
     GL_LUMINANCE_ALPHA) are not supported, however. Those who wish to emulate
     these functions may define	3D textures with r size	or t size of one, and
     pad the incoming pixel groups with	concocted blue or green	values.

     On	High Impact and	Maximum	Impact systems,	the effect of the current
     raster position's color is	not taken into account,	as if the color	is
     white, with full alpha.  This will	be fixed in the	next release.

     On	High Impact and	Maximum	Impact systems,	glPixelTexGenSGIX for
     glCopyPixels is not supported. This will be fixed in the next release.


SEE ALSO
     glTexEnv, glTexImage1D, glTexImage2D, glTexImage3DEXT, glTexImage4DSGIS,
     glTexParameter




























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