afunct(3G)

NAME
     afunct - specify alpha test function

FORTRAN SPECIFICATION
     subroutine afunct(ref, func)
     integer*4 ref, func

PARAMETERS
     ref    expects a reference value with which to compare source alpha at
            each pixel.  This value should be in the range 0 through 255.

     func   is a symbolic constant taken from the list below.  It identifies
            the alpha comparison function:  AFNEVE, AFLESS, AFEQUA, AFLEQU,
            AFGREA, AFNOTE, AFGEQU, and AFALWA.  AFALWA is the default.

DESCRIPTION
     afunct makes the drawing of pixels conditional on the relationship of the
     incoming alpha value with a reference constant value.  It is typically
     used to avoid updating either the color or the z bitplanes of a
     framebuffer pixel when the incoming pixel is completely transparent.
     Arguments ref and func specify the conditions under which the pixel will
     be drawn.  The incoming (source) alpha value is compared against ref with
     function func.  This comparison function is specified with the symbolic
     constants:

     AFNEVE     Never pass.

     AFNEVE     Pass if alpha is less than ref.

     AFNEVE     Pass if alpha is equal to ref.

     AFNEVE     Pass if alpha is less than or equal to ref.

     AFNEVE     Pass if alpha is greater than ref.

     AFNEVE     Pass if alpha is greater than or equal to ref.

     AFNEVE     Pass if alpha is not equal to ref.

     AFNEVE     Always pass.

     If the comparison passes, the incoming pixel is drawn (conditional on
     subsequent other tests like z-buffer).

     afunct can be called with arguments 0,AFNOTE to defeat drawing of
     completely transparent pixels.  This assumes that incoming alpha is
     proportional to pixel coverage, as it is when either points or linesm is
     being used.

     afunct testing follows scan conversion and texture mapping but precedes
     all other pixel tests.  Thus, if the test fails, color, zbuffer and
     stencil contents will not be modified.
     afunct operates on all pixel writes, including those resulting from the
     scan conversion of points, lines, and polygons, and from pixel write and
     copy operations. afunct does not affect screen clear operation, however.

SEE ALSO
     blendf

NOTES
     IRIS-4D G, GT, and GTX models, and the Personal Iris, do not support
     afunct.  Use getgde to determine what support is available for afunct.

BUGS
     On IRIS-4D VGX, XS, XS24, XZ, Elan and Extreme models, afunct cannot be
     enabled while stenci is being used.  Also on these models, only AFNOTE
     with ref equal to 0 and AFALWA are supported.