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glAlphaFunc — specify the alpha test function
void glAlphaFunc(GLenum func, GLclampf ref);
func
Specifies the alpha comparison function.
Symbolic constants
GL_NEVER,
GL_LESS,
GL_EQUAL,
GL_LEQUAL,
GL_GREATER,
GL_NOTEQUAL,
GL_GEQUAL, and
GL_ALWAYS are accepted. The initial value is GL_ALWAYS.
ref
Specifies the reference value that incoming alpha values are compared to.
This value is clamped to the range
The alpha test discards fragments depending on the outcome of a comparison
between an incoming fragment's alpha value and a constant reference value.
glAlphaFunc specifies the reference value and the comparison function.
The comparison is performed only if alpha testing is enabled. By
default, it is not enabled.
(See
glEnable and glDisable of GL_ALPHA_TEST.)
func and ref specify the conditions under which
the pixel is drawn.
The incoming alpha value is compared to ref
using the function specified by func.
If the value passes the comparison,
the incoming fragment is drawn
if it also passes subsequent stencil and depth buffer tests.
If the value fails the comparison,
no change is made to the frame buffer at that pixel location. The
comparison functions are as follows:
GL_NEVERNever passes.
GL_LESSPasses if the incoming alpha value is less than the reference value.
GL_EQUALPasses if the incoming alpha value is equal to the reference value.
GL_LEQUALPasses if the incoming alpha value is less than or equal to the reference value.
GL_GREATERPasses if the incoming alpha value is greater than the reference value.
GL_NOTEQUALPasses if the incoming alpha value is not equal to the reference value.
GL_GEQUALPasses if the incoming alpha value is greater than or equal to the reference value.
GL_ALWAYSAlways passes (initial value).
glAlphaFunc operates on all pixel write operations,
including those resulting from the scan conversion of points,
lines,
polygons,
and bitmaps,
and from pixel draw and copy operations.
glAlphaFunc does not affect screen clear operations.
GL_INVALID_ENUM is generated if func is not an accepted value.
GL_INVALID_OPERATION is generated if glAlphaFunc
is executed between the execution of glBegin
and the corresponding execution of glEnd.
glGet with argument GL_ALPHA_TEST_FUNC
glGet with argument GL_ALPHA_TEST_REF
glIsEnabled with argument GL_ALPHA_TEST
Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.