2003-2004
Silicon Graphics, Inc.
glAlphaFunc
3G
glAlphaFunc
glAlphaFunc
glAlphaFuncx
specify the alpha test function
C Specification
void glAlphaFunc
GLenum func
GLclampf ref
void glAlphaFuncx
GLenum func
GLclampx ref
Parameters
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 [0, 1], where 0 represents the lowest possible
alpha value and 1 the highest possible value. The initial
reference value is 0.
Description
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.
To enable and disable alpha testing, call
glEnable and
glDisable
with argument GL_ALPHA_TEST.
Alpha testing is initially disabled. When disabled, it is as if the comparison
always passes.
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_NEVER
Never passes.
GL_LESS
Passes if the incoming alpha value is less than the
reference value.
GL_EQUAL
Passes if the incoming alpha value is equal to the
reference value.
GL_LEQUAL
Passes if the incoming alpha value is less than or
equal to the reference value.
GL_GREATER
Passes if the incoming alpha value is greater than
the reference value.
GL_NOTEQUAL
Passes if the incoming alpha value is not equal to
the reference value.
GL_GEQUAL
Passes if the incoming alpha value is greater than
or equal to the reference value.
GL_ALWAYS
Always passes (initial value).
glAlphaFunc
operates on all pixel write operations, including those
resulting from the scan conversion of points, lines, and
polygons.
glAlphaFunc
does not affect
glClear.
Errors
GL_INVALID_ENUM is generated if
func is not an accepted value.
See Also
glBlendFunc,
glClear,
glDepthFunc,
glEnable,
glStencilFunc
Copyright
Copyright 2003-2004
Silicon Graphics, Inc. This document is licensed under the SGI
Free Software B License. For details, see
https://khronos.org/registry/OpenGL-Refpages/LICENSES/LicenseRef-FreeB.txt.