afunction(3G)afunction(3G)NAMEafunction - specify alpha test function
C SPECIFICATION
void afunction(ref, func)
long 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: AF_NEVER, AF_LESS, AF_EQUAL,
AF_LEQUAL, AF_GREATER, AF_NOTEQUAL, AF_GEQUAL, and AF_ALWAYS.
AF_ALWAYS is the default.
DESCRIPTIONafunction 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:
AF_NEVER Never pass.
AF_LESS Pass if alpha is less than ref.
AF_EQUAL Pass if alpha is equal to ref.
AF_LEQUAL Pass if alpha is less than or equal to ref.
AF_GREATER Pass if alpha is greater than ref.
AF_GEQUAL Pass if alpha is greater than or equal to ref.
AF_NOTEQUAL Pass if alpha is not equal to ref.
AF_ALWAYS Always pass.
If the comparison passes, the incoming pixel is drawn (conditional on
subsequent other tests like z-buffer).
afunction can be called with arguments 0,AF_NOTEQUAL to defeat drawing of
completely transparent pixels. This assumes that incoming alpha is
proportional to pixel coverage, as it is when either pointsmooth or
linesmooth is being used.
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afunction(3G)afunction(3G)afunction 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.
afunction operates on all pixel writes, including those resulting from
the scan conversion of points, lines, and polygons, and from pixel write
and copy operations. afunction does not affect screen clear operation,
however.
SEE ALSO
blendfunction
NOTES
IRIS-4D G, GT, and GTX models, and the Personal Iris, do not support
afunction. Use getgdesc to determine what support is available for
afunction.
BUGS
On IRIS-4D VGX, XS, XS24, XZ, Elan and Extreme models, afunction cannot
be enabled while stencil is being used. Also on these models, only
AF_NOTEQUAL with ref equal to 0 and AF_ALWAYS are supported.
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