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  repre-
	     sents 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 refer-
       ence 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 speci-
       fied 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 bit-
       maps, 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

																   GLALPHAFUNC(3G)