afunction(3G)							 afunction(3G)

NAME    [Toc]    [Back]

     afunction - specify alpha test function

C SPECIFICATION    [Toc]    [Back]

     void afunction(ref, func)
     long ref, func;

PARAMETERS    [Toc]    [Back]

     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.

DESCRIPTION    [Toc]    [Back]

     afunction 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    [Toc]    [Back]

     blendfunction

NOTES    [Toc]    [Back]

     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    [Toc]    [Back]

     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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