glAlphaFunc - specify the alpha test function
void glAlphaFunc( GLenum func,
GLclampf ref )
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 default function is
ref 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 default reference is 0.
The alpha test discards fragments depending on the outcome of a comparison
between the incoming fragment's alpha value and a constant reference value.
glAlphaFunc specifies the reference and comparison function. The
comparison is performed only if alpha testing is 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 comparison passes, the incoming fragment is drawn,
conditional on subsequent stencil and depth buffer tests. If the comparison
fails, 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
GL_EQUAL Passes if the incoming alpha value is equal to the
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
GL_NOTEQUAL Passes if the incoming alpha value is not equal to the
GL_GEQUAL Passes if the incoming alpha value is greater than or
equal to the reference value.
GL_ALWAYS Always passes.
glAlphaFunc operates on all pixel writes, 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
Alpha testing is done 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 execution of glEnd.
glGet with argument GL_ALPHA_TEST_FUNC
glGet with argument GL_ALPHA_TEST_REF
glIsEnabled with argument GL_ALPHA_TEST
glBlendFunc, glClear, glDepthFunc, glEnable, glStencilFunc
Fri Dec 6 11:18:03 EST 1996
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