glStencilFunc	- set function and reference value for stencil
testing

glStencilFunc
	glStencilFunc(func, ref, mask) -> None

void glStencilFunc( GLenum func,
                    GLint ref,
                    GLuint mask )

func	Specifies the test function.  Eight tokens are valid:
      GL_NEVER, GL_LESS, GL_LEQUAL, GL_GREATER, GL_GEQUAL,
      GL_EQUAL, GL_NOTEQUAL, and GL_ALWAYS. The initial
      value is GL_ALWAYS.

ref	Specifies the reference	value for the stencil test.
      ref is clamped to the range [0,2n-1], where n is the
      number of bitplanes in the stencil buffer. The initial
      value is 0.

mask	Specifies a mask that is ANDed with both the reference
      value and the stored stencil value when	the test is
      done. The initial value	is all 1's.

Stenciling, like depth-buffering, enables and	disables
drawing on a per-pixel basis.	 You draw into the stencil
planes using GL drawing primitives, then render geometry and
images, using	the stencil planes to mask out portions	of the
screen.  Stenciling is typically used	in multipass rendering
algorithms to	achieve	special	effects, such as decals,
outlining, and constructive solid geometry rendering.

The stencil test conditionally eliminates a pixel based on
the outcome of a comparison between the reference value and
the value in the stencil buffer.  To enable and disable the
test,	call glEnable and glDisable with argument
GL_STENCIL_TEST.  To specify actions based on	the outcome of
the stencil test, call glStencilOp.

func is a symbolic constant that determines the stencil
comparison function.	It accepts one of eight	values,	shown
in the following list.  ref is an integer reference value
that is used in the stencil comparison.  It is clamped to
the range [0,2n-1], where n is the number of bitplanes in
the stencil buffer.  mask is bitwise ANDed with both the
reference value and the stored stencil value,	with the ANDed
values participating in the comparison.

If stencil represents	the value stored in the	corresponding
stencil buffer location, the following list shows the	effect
of each comparison function that can be specified by func.
Only if the comparison succeeds is the pixel passed through
to the next stage in the rasterization process (see
glStencilOp).	 All tests treat stencil values	as unsigned
integers in the range	[0,2n-1], where	n is the number	of
bitplanes in the stencil buffer.

The following	values are accepted by func:

GL_NEVER	    Always fails.

GL_LESS	    Passes if (	ref & mask ) < ( stencil &
                  mask ).

GL_LEQUAL	    Passes if (	ref & mask ) < ( stencil &
                  mask ).

GL_GREATER	    Passes if (	ref & mask ) > ( stencil &
                  mask ).

GL_GEQUAL	    Passes if (	ref & mask ) > ( stencil &
                  mask ).

GL_EQUAL	    Passes if (	ref & mask ) = ( stencil &
                  mask ).

GL_NOTEQUAL	    Passes if (	ref & mask ) /	( stencil &
                  mask ).

GL_ALWAYS	    Always passes.

Initially, the stencil test is disabled.  If there is	no
stencil buffer, no stencil modification can occur and	it is
as if	the stencil test always	passes.

GL_INVALID_ENUM is generated if func is not one of the eight
accepted values.

GL_INVALID_OPERATION is generated if glStencilFunc is
executed between the execution of glBegin and	the
corresponding	execution of glEnd.

glGet	with argument GL_STENCIL_FUNC
glGet	with argument GL_STENCIL_VALUE_MASK
glGet	with argument GL_STENCIL_REF
glGet	with argument GL_STENCIL_BITS
glIsEnabled with argument GL_STENCIL_TEST

glAlphaFunc, glBlendFunc, glDepthFunc, glEnable,
glIsEnabled, glLogicOp, glStencilOp