glAlphaFunc
[New
- Windows 95, OEM Service Release 2]
The glAlphaFunc
function specifies the alpha test function.
void glAlphaFunc(
|
GLenum func, |
|
|
GLclampf ref |
|
|
); |
|
Parameters
func
The alpha
comparison function. The following are the accepted symbolic constants and
their meanings.
|
Symbolic
Constant |
Meaning |
|
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. This is the default. |
ref
The reference
value to which incoming alpha values are compared. 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.
Remarks
The alpha
test discards fragments depending on the outcome of a comparison between the
incoming fragments alpha values and a constant reference value. The glAlphaFunc
function specifies the reference and comparison function. The comparison is
performed only if alpha testing is enabled. (For more information on
GL_ALPHA_TEST, see glEnable.)
The func
and ref parameters 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 glAlphaFunc
function 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. The glAlphaFunc function does not affect screen clear
operations.
Alpha testing
is done only in RGBA mode.
The following
functions retrieve information related to the glAlphaFunc function:
glGet
with argument GL_ALPHA_TEST_FUNC
glGet with
argument GL_ALPHA_TEST_REF
glIsEnabled with argument GL_ALPHA_TEST
Error Codes
The following
are the error codes generated and their conditions.
|
Error
Code |
Condition |
|
GL_INVALID_ENUM
|
func was not an accepted value. |
|
GL_INVALID_OPERATION
|
glAlphaFunc was called between a call to glBegin and the
corresponding call to glEnd. |
See Also