afunct(3G)afunct(3G)NAMEafunct - 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.
DESCRIPTIONafunct 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.
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afunct(3G)afunct(3G)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.
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