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       glStencilOp - set stencil test actions

       delim $$

       void glStencilOp( GLenum	fail,
			 GLenum	zfail,
			 GLenum	zpass )

       fail   Specifies	 the  action to	take when the stencil test fails.  Six
	      symbolic constants are accepted: GL_KEEP,	 GL_ZERO,  GL_REPLACE,
	      GL_INCR, GL_DECR,	and GL_INVERT. The initial value is GL_KEEP.

       zfail  Specifies	 the  stencil action when the stencil test passes, but
	      the depth	test fails.  zfail accepts the same symbolic constants
	      as fail. The initial value is GL_KEEP.

       zpass  Specifies	 the stencil action when both the stencil test and the
	      depth test pass, or when the  stencil  test  passes  and	either
	      there is no depth	buffer or depth	testing	is not enabled.	 zpass
	      accepts the same symbolic	constants as fail. The	initial	 value
	      is GL_KEEP.

       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 de-
       cals, outlining,	and constructive solid geometry	rendering.

       The stencil test	conditionally eliminates a pixel based on the  outcome
       of a comparison between the value in the	stencil	buffer and a reference
       value. To enable	and disable the	test, call glEnable and	glDisable with
       argument	GL_STENCIL_TEST; to control it,	call glStencilFunc.

       glStencilOp  takes  three  arguments  that indicate what	happens	to the
       stored stencil value while stenciling is	enabled.  If the stencil  test
       fails,  no  change  is  made to the pixel's color or depth buffers, and
       fail specifies what happens to the stencil buffer contents.   The  fol-
       lowing six actions are possible.

       GL_KEEP	       Keeps the current value.

       GL_ZERO	       Sets the	stencil	buffer value to	0.

       GL_REPLACE      Sets  the  stencil buffer value to ref, as specified by

       GL_INCR	       Increments the current stencil buffer value.  Clamps to
		       the maximum representable unsigned value.

       GL_DECR	       Decrements the current stencil buffer value.  Clamps to

       GL_INVERT       Bitwise inverts the current stencil buffer value.

       Stencil buffer values are treated as unsigned  integers.	  When	incre-
       mented  and  decremented,  values  are  clamped to 0 and	$2 sup n - 1$,
       where $n$ is the	value returned by querying GL_STENCIL_BITS.

       The other two arguments to glStencilOp specify stencil  buffer  actions
       that depend on whether subsequent depth buffer tests succeed (zpass) or
       fail (zfail) (see
       glDepthFunc).  The actions are specified	using the  same	 six  symbolic
       constants  as  fail.  Note that zfail is	ignored	when there is no depth
       buffer, or when the depth buffer	is not enabled.	 In these cases,  fail
       and  zpass  specify  stencil  action  when  the	stencil	test fails and
       passes, respectively.

       Initially the stencil test is disabled.	If there is no stencil buffer,
       no stencil modification can occur and it	is as if the stencil tests al-
       ways pass, regardless of	any call to glStencilOp.

       GL_INVALID_ENUM is generated if fail, zfail,  or	 zpass	is  any	 value
       other than the six defined constant values.

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

       glGet with argument GL_STENCIL_FAIL
       glGet with argument GL_STENCIL_PASS_DEPTH_PASS
       glGet with argument GL_STENCIL_PASS_DEPTH_FAIL
       glGet with argument GL_STENCIL_BITS
       glIsEnabled with	argument GL_STENCIL_TEST

       glAlphaFunc, glBlendFunc, glDepthFunc, glEnable,	glLogicOp,  glStencil-



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