glSamplePatternSGIS(3G)

NAME
     glSamplePatternSGIS - set pattern for multisampling


C SPECIFICATION
     void glSamplePatternSGIS( GLenum pattern )


PARAMETERS
     pattern  A symbolic constant that indicates the sampling pattern to be
              used when multisampling is enabled.  The allowable values are
              GL_1PASS_SGIS, GL_2PASS_0_SGIS, GL_2PASS_1_SGIS,
              GL_4PASS_0_SGIS, GL_4PASS_1_SGIS, GL_4PASS_2_SGIS, and
              GL_4PASS_3_SGIS.

DESCRIPTION
     glSamplePatternSGIS is part of the SGIS_multisample extension.  This
     extension provides multisampling, a mechanism for antialiasing all GL
     primitives: points, lines, polygons, bitmaps, and images.  Multisample
     antialiasing is most valuable for rendering polygons, because it requires
     no sorting for hidden surface elimination, and it correctly handles
     adjacent polygons, object silhouettes, and even intersecting polygons.
     If only points or lines are being rendered, the "smooth" antialiasing
     mechanism provided by the base GL may result in a higher quality image.
     This extension is designed to allow multisample and smooth antialiasing
     techniques to be alternated during the rendering of a single scene.

     The technique is to sample all primitives multiple times at each pixel.
     The color sample values are resolved to a single, displayable color each
     time a pixel is updated, so the antialiasing appears to be automatic at
     the application level.  Because each sample includes depth and stencil
     information, the depth and stencil functions perform equivalently to the
     single-sample mode.

     An additional buffer, called the multisample buffer, is required.  Pixel
     sample values, including color, depth, and stencil values, are stored in
     this buffer.  When the frame buffer includes a multisample buffer, it
     does not also include separate depth or stencil buffers, even if the
     multisample buffer does not store depth or stencil values.  Color buffers
     (left/right, front/ back, and aux) do coexist with the multisample
     buffer, however.

     The locations of the sample points are relatively static.  They change
     only when glSamplePatternSGIS is called, and are a repeatable function of
     the parameter pattern.  The default pattern GL_1PASS_SGIS is designed to
     produce a well antialiased result in a single rendering pass.  Patterns
     GL_2PASS_0_SGIS and GL_2PASS_1_SGIS together specify twice the number of
     sample points per pixel.  It is intended that a scene be completely
     rendered using pattern GL_2PASS_0_SGIS, then completely rendered again
     using GL_2PASS_1_SGIS.  When these two images are averaged using the
     accumulation buffer, the result is as if a single pass had been rendered
     with a pattern of twice the number of sample points.  Patterns
     GL_4PASS_0_SGIS, GL_4PASS_1_SGIS, GL_4PASS_2_SGIS, and GL_4PASS_3_SGIS
     together define a pattern of four times the number of sample points. They
     can be used to accumulate an image after four complete rendering passes.


NOTES
     It is not possible to query the actual sample locations of a pixel.

     Some sample locations may lie outside the unit square that is considered
     to bound the pixel.

     glSamplePatternSGIS 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 glSamplePatternSGIS is executed
     between the execution of glBegin and the corresponding execution of
     glEnd.


ASSOCIATED GETS
     glGet with argument GL_SAMPLE_PATTERN_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
     glSampleMaskSGI, glPushAttrib, glXGetConfig, glXChooseVisual, glEnable
     with argument GL_MULTISAMPLE_SGIS.