glAlphaFunc(3G)		       OpenGL Reference		       glAlphaFunc(3G)

NAME    [Toc]    [Back]

     glAlphaFunc - specify the alpha test function

C SPECIFICATION    [Toc]    [Back]

     void glAlphaFunc( GLenum func,
		       GLclampf	ref )

PARAMETERS    [Toc]    [Back]

     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    [Toc]    [Back]

     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.




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glAlphaFunc(3G)		       OpenGL Reference		       glAlphaFunc(3G)



     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    [Toc]    [Back]

     Alpha testing is performed	only in	RGBA mode.

ERRORS    [Toc]    [Back]

     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    [Toc]    [Back]

     glGet with	argument GL_ALPHA_TEST_FUNC
     glGet with	argument GL_ALPHA_TEST_REF
     glIsEnabled with argument GL_ALPHA_TEST

SEE ALSO    [Toc]    [Back]

     glBlendFunc, glClear, glDepthFunc,	glEnable, glStencilFunc


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