glAlphaFunc(3G)		       OpenGL Reference		       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 default function	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	default	reference is 0.


DESCRIPTION
     The alpha test discards fragments depending on the	outcome	of a
     comparison	between	the incoming fragment's	alpha value and	a constant
     reference value.	glAlphaFunc specifies the reference and	comparison
     function.	The comparison is performed only if alpha testing is 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 comparison passes, the incoming fragment is drawn,
     conditional on subsequent stencil and depth buffer	tests. If the
     comparison	fails, 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.




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



     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.

     glAlphaFunc operates on all pixel writes, 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 done 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

























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