GLALPHAFUNC(3G) UNIX Programmer's Manual GLALPHAFUNC(3G)
glAlphaFunc - specify the alpha test function
void glAlphaFunc( GLenum func,
GLclampf ref )
func Specifies the alpha comparison function. Symbolic con-
stants 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 ini-
tial reference value is 0.
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 com-
parison, 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 fol-
lows:
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.
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GLALPHAFUNC(3G) UNIX Programmer's Manual GLALPHAFUNC(3G)
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, includ-
ing 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 opera-
tions.
Alpha testing is performed only in RGBA mode.
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 exe-
cution of glEnd.
glGet with argument GL_ALPHA_TEST_FUNC
glGet with argument GL_ALPHA_TEST_REF
glIsEnabled with argument GL_ALPHA_TEST
glBlendFunc(3G), glClear(3G), glDepthFunc(3G), glEnable(3G),
glStencilFunc(3G)
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