glAlphaFunc(3G)glAlphaFunc(3G)NAMEglAlphaFunc - specify the alpha test function
SYNOPSIS
void glAlphaFunc(
GLenum func,
GLclampf ref );
PARAMETERS
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. 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 initial reference
value is 0.
DESCRIPTION
The alpha test discards fragments depending on the outcome of a compar‐
ison between an incoming fragment's alpha value and a constant refer‐
ence value. glAlphaFunc() specifies the reference value and the com‐
parison 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: 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 value. 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 reference value. Always passes (initial value).
glAlphaFunc() operates on all pixel write operations, including those
resulting from the scan conversion of points, lines, polygons, and bit‐
maps, and from pixel draw and copy operations. glAlphaFunc() does not
affect screen clear operations.
NOTES
Alpha testing is performed only in RGBA mode.
ERRORS
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().
ASSOCIATED GETSglGet() with argument GL_ALPHA_TEST_FUNC
glGet() with argument GL_ALPHA_TEST_REF
glIsEnabled() with argument GL_ALPHA_TEST
SEE ALSOglBlendFunc(3), glClear(3), glDepthFunc(3), glEnable(3), glStencil‐
Func(3)glAlphaFunc(3G)
