GLALPHAFUNC(3G)

NAME
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 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).

glAlphaFunc 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. 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 GETS

glGet() with argument GL_ALPHA_TEST_FUNC

glGet() with argument GL_ALPHA_TEST_REF

glIsEnabled() with argument GL_ALPHA_TEST

SEE ALSO

glBlendFunc(), glClear(), glDepthFunc(), glEnable(), glStencilFunc()

COPYRIGHT

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/.