glAlphaFunc -	specify	the alpha test function

glAlphaFunc
	glAlphaFunc(func, ref) -> None

void glAlphaFunc( GLenum func,
                  GLclampf ref )

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.

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.

Alpha	testing	is performed only in RGBA mode.

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.

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

glBlendFunc, glClear,	glDepthFunc, glEnable, glStencilFunc