glAlphaFunc

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 com-
       parison between an incoming fragment’s alpha value and a constant ref-
       erence  value.  glAlphaFunc specifies the reference value and the com-
       parison 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
       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)



							      GLALPHAFUNC(3G)