glAlphaFunc man page on RedHat
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GLALPHAFUNC(3G) OpenGL Manual GLALPHAFUNC(3G)
NAME
glAlphaFunc - specify the alpha test function
C SPECIFICATION
void glAlphaFunc(GLenum func, GLclampf 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. 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
glBlendFunc(), glClear(), glDepthFunc(), glEnable(), glStencilFunc()
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/.
AUTHORS
opengl.org
opengl.org 03/16/2013 GLALPHAFUNC(3G)
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