Name

glAlphaFunc — specify the alpha test function

C Specification

void glAlphaFunc(GLenum func,
 GLclampf ref);
 
void glAlphaFuncx(GLenum func,
 GLclampx 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, 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. To enable and disable alpha testing, call glEnable and glDisable with argument GL_ALPHA_TEST. Alpha testing is initially disabled. When disabled, it is as if the comparison always passes.

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).

glAlphaFunc operates on all pixel write operations, including those resulting from the scan conversion of points, lines, and polygons. glAlphaFunc does not affect glClear.

Errors

GL_INVALID_ENUM is generated if func is not an accepted value.

See Also

glBlendFunc, glClear, glDepthFunc, glEnable, glStencilFunc

Copyright

Copyright © 2003-2004 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.