glalphafunc man page on SunOS

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GLALPHAFUNC(3gl)					      GLALPHAFUNC(3gl)

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 com‐
	     pared  to.	 This value is clamped to the range 0 through 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 compar‐
       ison between an incoming fragment's alpha value and a  constant	refer‐
       ence value.  glAlphaFunc specifies the reference value and the compari‐
       son 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	 frag‐
       ment  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

				   15 Mar 97		      GLALPHAFUNC(3gl)
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