- specify the alpha test function
C SPECIFICATION
PARAMETERS
_param1 Specifies the alpha comparison function. Sym-
bolic 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.
_param2 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 compari-
son is performed only if alpha testing is enabled. By
default, it is not enabled. (See glEnable and glDisable
of GL_ALPHA_TEST.)
_param1 and _param2 specify the conditions under which the
pixel is drawn. The incoming alpha value is compared to
_param2 using the function specified by _param1. 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.
Alpha testing is performed only in RGBA mode.
ERRORS
GL_INVALID_ENUM is generated if _param1 is not an accepted
value.
GL_INVALID_OPERATION is generated if glAlphaFunc is exe-
cuted between the execution of glBegin and the correspond-
ing 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
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