glAlphaFunc: specify the alpha test function.
C Specification |
Associated Gets |
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_GEQUAL, and GL_ALWAYS are accepted. The
initial value is GL_ALWAYS.
- Specifies the reference value that incoming alpha values are compared to.
This value is clamped to the range 0 through 1, where 0 represents the
lowest possible alpha value and 1 the highest possible value. The initial
reference value is 0.
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:
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
Alpha testing is performed only in RGBA mode.
- 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 reference value.
- 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
- Always passes (initial value).
glGet with argument
- 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.
glGet with argument
glIsEnabled with argument