multis(3G)

NAME
     multis - specifies the use of the multisample buffer

FORTRAN 77 SPECIFICATION
     subroutine multis(bool)
     logical bool

PARAMETERS
     bool

     bool   is either .TRUE. or .FALSE., indicating whether rendering is to
            make use of the multisample buffer or not.  By default multis is
            TRUE.

DESCRIPTION
     When multisample is enabled, and multisample buffers exist in the current
     framebuffer configuration in the current drawmode, rendered primitives
     directly affect the samples in the multisample buffer.  Immediately after
     the multisample locations at a pixel are modified, the front and/or back
     framebuffer colors are written with the "average" value of the
     multisample color values.  No change is made to the standard z buffer or
     stencil buffer that may be associated with the color buffers, and their
     contents do not affect rendering operation.

     Polygons are sampled into the multisample buffer as much as they are
     sampled into the standard single-sample buffer.  A single color value is
     computed for the entire pixel, regardless of the number of samples at
     that pixel.  Each multisample location is then written with this color,
     if and only if, it is geometrically within the exact polygon boundary.
     If the Z buffer is enabled, the correct depth value at each multisample
     location is computed and used to determine whether that sample should be
     written or not.  If stencil is enabled, the test is performed at each
     multisample location.  Framebuffer modes alpha test, blending, dithering,
     and writemask affect the modification of the individual multisamples, and
     have no effect on the transfer of the "average" color value to the front
     or back color buffers.  Alternately, buffer enables frontb, backbu,
     leftbu, rightb, and zdraw have no effect on the modification of the
     individual multisample locations; they affect only the transfer of the
     "average" color value.

     Polygon pattern and line style pattern bits apply equally to all
     multisample locations at a pixel.  All sample locations are considered
     for modification if the pattern bit is one.  None are considered if the
     pattern bit is zero.

     Lines are sampled into the multisample buffer as rectangles centered
     around the exact zero-area segment.  The rectangle width is equal to the
     current linewidth. Its length is exactly equal to the length of the
     segment.  The rectangles of colinear, abutting line segments abut
     exactly, so no multisamples are missed or drawn twice near the shared
     vertex.

     Points are sampled into the multisample buffer as circles centered around
     the exact point location.  The circle diameter is equal to the current
     pointsize.

     Polysmooth, linesmooth, and pointsmooth, must be disabled while
     multisample is enabled, or rendering results are undefined.

     Text is sampled into the multisample buffer by converting each '1' in the
     character mask into a 1x1 square, which is then sampled just like a
     polygon.

     During pixel operations either all multisample locations at a pixel are
     sampled or all are not.

     There are no special clear commands for the multisample buffer.  Rather,
     the standard clear, zclear, sclear, and czclea commands affect the
     multisample buffer as much as they affect the standard color, stencil,
     and z buffers.  Clear modifies the enabled color buffers, and always
     modifies the color portion of each multisample location.  zclear operates
     on all multisample z locations.  sclear operates on all multisample
     stencil locations.  czclea behaves like clear and zclear, except that the
     z value is specified.

     When multisample is disabled, but the system is configured for
     multisample operation, rendering neither affects the multisample buffer,
     nor is it a function of the contents of the multisample buffer.  It is as
     though there were no multisample buffer at all.  zbuffer, if configured
     and enabled, tests and updates the standard z buffer, not the zbuffer
     locations in the multisample buffer.  Likewise stencil, if configured and
     enabled, tests and updates the standard stencil buffer, not the stencil
     locations in the multisample buffer.

     Polysmooth, linesmooth, and pointsmooth are well defined when multisample
     is false.

     Multisample state is maintained but ignored if no multisample buffer is
     configured.  Because it is not possible to allocate a multisample buffer
     in color index mode, multisample is always ignored in color index mode.

     The multisample buffer is a part of the normal framebuffer.  multisample
     should be called only while drawmode is NORMAL, and it affects rendering
     only while drawmode is NORMAL.

SEE ALSO
     msalph, msmask, mspatt, mssize

BUGS
     On the IRIS-4D RealityEngine model the values of frontbuffer, backbuffer,
     leftbuffer, and rightbuffer should not be changed between the time clear
     or czclea are called and the time that the rendered image is complete.
     At least one of the color buffers must be enabled during all use of the
     multisample buffer.
     Points are in fact sampled into the multisample buffer as squares where
     the squre width is equal to the current pointsize.  This will be fixed in
     a future release.