glTagSampleBufferSGIX(3G)      OpenGL Reference	     glTagSampleBufferSGIX(3G)



NAME
     glTagSampleBufferSGIX - mark all samples of a multisample buffer


C SPECIFICATION
     void glTagSampleBufferSGIX( void )



DESCRIPTION
     When using	a multisample buffer on	some systems (see Machine Dependencies
     below), glTagSampleBufferSGIX can be used with some restrictions as a
     faster alternative	to clearing the	depth buffer using glClear.  On	these
     system, each sample in the	multisample buffer carries a tag that can be
     either set	or cleared.  glTagSampleBufferSGIX sets	the tags of all	the
     samples.  When a tag of a sample is set, a	depth test against that	sample
     always succeeds.  Note, however, that the stencil test (if	enabled) still
     uses the stencil value that is stored at the sample.  When	a color	is
     written into a sample, its	tag is cleared.	 Depth tests against a sample
     with a cleared tag	use the	depth value stored at the sample.

     Initially the state of the	tags of	all samples is undefined.  Clearing
     the main color buffer clears all the tags.	 For a given pixel, the	color
     values stored in the sample buffer	are resolved into the main color
     buffer only when the tags of all samples of that pixel are	cleared.
     Thus, when	using glTagSampleBufferSGIX, it	is the responsibility of the
     program to	ensure that the	tags of	all samples are	cleared	(i.e., that a
     color is stored into each sample least once) before the frame is
     completed.

     Using glTagSampleBufferSGIX is much faster	than
     glClear(GL_DEPTH_BUFFER_BIT), but it does not have	the same effect	(i.e.,
     no	change is made to the contents of the depth buffer).

     The result	of blending a color into a sample whose	tag is set is
     undefined.	 Therefore, blending should be disabled	when writing into
     samples with set tags.

     If	a multisample buffer is	not available, glTagSampleBufferSGIX has no
     effect.

NOTES
     Programs that guarantee that a color value	is stored into every sample of
     each frame	can use	glTagSampleBufferSGIX instead of clearing the depth
     and the main color	buffer(s), resulting in	increased performance.

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





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glTagSampleBufferSGIX(3G)      OpenGL Reference	     glTagSampleBufferSGIX(3G)



MACHINE	DEPENDENCIES
     Multisampling and sample tags are supported only on RealityEngine,
     RealityEngine2, and VTX systems.  Sample tags are experimental and	might
     not be supported in future	systems.


SEE ALSO
     glBlendFunc glClear glEnable glSampleMaskSGIS, glSamplePatternSGIS,















































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