glSampleMaskSGIS(3G) OpenGL Reference glSampleMaskSGIS(3G)
NAME
glSampleMaskSGIS - define mask to modify multisampled pixel fragments
C SPECIFICATION
void glSampleMaskSGIS( GLclampf value,
GLboolean invert )
PARAMETERS
value Specifies coverage of the modification mask. Clamped to the
range [0, 1]; zero implies no coverage, and one implies full
coverage.
invert GLFALSE to use the modification mask implied by value; GLTRUE
to use the bitwise inverse of that mask.
DESCRIPTION
glSampleMaskSGIS defines a mask to modify the coverage of multisampled
pixel fragments. This capability is used for antialiased screen-door
transparency and smooth transitions between two renderings of an object
(often for level-of-detail management in simulation systems).
When multisampling is enabled (see glEnable with argument
GLMULTISAMPLESGIS) a ``fragment mask'' is computed for each fragment
generated by a primitive. This mask reflects the amount of the pixel
covered by the fragment, and determines the frame buffer samples that may
be affected by the fragment.
If conversion of alpha values to masks is enabled (glEnable with argument
GLSAMPLEALPHATOMASKSGIS), the fragment alpha value is used to
generate a temporary modification mask which is then ANDed with the
fragment mask. One way to interpret this is as a form of dithering: a
multivalued alpha (coverage or opacity) for the whole fragment is
converted to simple binary values of coverage at many locations (the
samples).
After conversion of alpha values to masks, if replacement of alpha values
is enabled (glEnable with argument GLSAMPLEALPHATOONESGIS), the
fragment's alpha is set to the maximum allowable value.
Finally, if fragment mask modification is enabled (glEnable with argument
GLSAMPLEMASKSGIS), glSampleMaskSGIS defines an additional modification
mask. value is used to generate a modification mask in much the same way
alpha was used above. If invert is GLTRUE, then the modification mask
specified by value will be inverted. The final modification mask will
then be ANDed with the fragment mask resulting from the previous steps.
This can be viewed as an ``override'' control that selectively fades the
effects of multisampled fragments.
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glSampleMaskSGIS(3G) OpenGL Reference glSampleMaskSGIS(3G)
Note that glSampleMaskSGIS(value, GLTRUE) is not necessarily equivalent
to glSampleMaskSGIS(1.0-value, GLFALSE); due to round-off and other
issues, complementing the coverage will not necessarily yield an inverted
modification mask.
NOTES
glSampleMaskSGIS is part of the SGISmultisample extension, not part of
the core GL command set. If GLSGISmultisample is included in the
string returned by glGetString, when called with argument GLEXTENSIONS,
extension SGISmultisample is supported by the connection.
ERRORS
GLINVALIDOPERATION is generated if glSampleMaskSGIS is executed between
the execution of glBegin and the corresponding execution of glEnd.
ASSOCIATED GETS
glGet with argument GLSAMPLEMASKVALUESGI, or
GLSAMPLEMASKINVERTSGI.
glIsEnabled with argument GLMULTISAMPLESGI, GLSAMPLEALPHATOMASK,
GLSAMPLEALPHATOONESGI, GLSAMPLEMASKSGI.
MACHINE DEPENDENCIES
Multisampling is supported only on RealityEngine, RealityEngine2, and VTX
and InfiniteReality systems. Currently it can be used with windows of
multisample-capable Visual types, but not with pixmaps.
SEE ALSO
glSamplePatternSGIS
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