glAlphaFunc man page on OpenBSD

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GLALPHAFUNC(3G)						       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(3G), glClear(3G), glDepthFunc(3G), glEnable(3G),
       glStencilFunc(3G)

								 March 1, 2011
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