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	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 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