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glAlphaFunc - specify the alpha test function
glAlphaFunc( GLenum func,
GLclampf ref )
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.)
- 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.
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.
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:
- Never passes.
- Passes if the incoming alpha value is less than the reference value.
- Passes if the incoming alpha value is equal to the reference value.
- Passes if the incoming alpha value is less than or equal to the reference
- Passes if the incoming alpha value is greater than the
- Passes if the incoming alpha value is not equal
to the reference value.
- Passes if the incoming alpha value is
greater than or equal to the reference value.
- Always passes (initial
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
Alpha testing is performed only in RGBA mode.
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.
glGet with argument GL_ALPHA_TEST_FUNC
glGet with argument GL_ALPHA_TEST_REF
glIsEnabled with argument GL_ALPHA_TEST
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