Gl.AlphaFunc (gb.opengl)
Static Sub AlphaFunc ( Function As Integer, Reference As Float )
Specify the alpha test function.
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
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.
Gl.AlphaFunc 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
Gl.Enable and
Gl.Disable 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).
Gl.AlphaFunc 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.
Gl.AlphaFunc 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
Gl.AlphaFunc
is executed between the execution of
Gl.Begin
and the corresponding execution of
Gl.End.
Associated Gets
Gl.Get with argument Gl.ALPHA_TEST_FUNC
Gl.Get with argument Gl.ALPHA_TEST_REF
Gl.IsEnabled with argument Gl.ALPHA_TEST
See Also
Gl.BlendFunc,
Gl.Clear,
Gl.DepthFunc,
Gl.Enable,
Gl.StencilFunc
Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB.
➡ See original documentation