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.
MACHINE DEPENDENCIES
     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,