Signature
Parameters
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. 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_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,
polygons,
and bitmaps,
and from pixel draw and copy operations.
glAlphaFunc
does not affect screen clear operations.
Notes
Alpha testing is performed only in RGBA mode.
Errors
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
.
Associated Gets
glGet
with argument
GL_ALPHA_TEST_FUNC
glGet
with argument
GL_ALPHA_TEST_REF
glIsEnabled
with argument
GL_ALPHA_TEST
See Also
Copyright
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/
.
Sample Code References
The following code samples have been found which appear to reference the functions described here. Take care that the code may be old, broken or not even use PyOpenGL.
glAlphaFunc
