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GLALPHAFUNC(3G) 			  OpenGL Manual 			  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 (C) 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/.

AUTHORS
       opengl.org

opengl.org				    06/10/2014				  GLALPHAFUNC(3G)
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