HP c3700 - Workstation, Reference Manual, Page: 13 / 486

A
glAlphaFunc
Chapter 1
13
glAlphaFunc
glAlphaFunc:
specify the alpha test function.
C Specification
void glAlphaFunc(
GLenum func,
GLclampf ref)
Parameters
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
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 through 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 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
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.
GL_GEQUAL Passes if the incoming alpha value is greater than or equal to the
reference value.
GL_ALWAYS Always passes (initial value).