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  GL_FALSE to use the modification mask implied by value; GL_TRUE
	     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
     GL_MULTISAMPLE_SGI) 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
     GL_SAMPLE_ALPHA_TO_MASK_SGIS), 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	GL_SAMPLE_ALPHA_TO_ONE_SGIS), the
     fragment's	alpha is set to	the maximum allowable value.

     Finally, if fragment mask modification is enabled (glEnable with argument
     GL_SAMPLE_MASK_SGIS), 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 GL_TRUE, 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, GL_TRUE)	is not necessarily equivalent
     to	glSampleMaskSGIS(1.0-value, GL_FALSE); due to round-off	and other
     issues, complementing the coverage	will not necessarily yield an inverted
     modification mask.


NOTES
     glSampleMaskSGIS is part of the SGIS_multisample extension, not part of
     the core GL command set.  If GL_SGIS_multisample is included in the
     string returned by	glGetString, when called with argument GL_EXTENSIONS,
     extension SGIS_multisample	is supported by	the connection.


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

ASSOCIATED GETS
     glGet with	argument GL_SAMPLE_MASK_VALUE_SGI, or
     GL_SAMPLE_MASK_INVERT_SGI.
     glIsEnabled with argument GL_MULTISAMPLE_SGI, GL_SAMPLE_ALPHA_TO_MASK,
     GL_SAMPLE_ALPHA_TO_ONE_SGI, GL_SAMPLE_MASK_SGI.


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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