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     <STRONG>NAME</STRONG>
	  <STRONG>glStencilOp</STRONG> -	set stencil test actions


     <STRONG>C</STRONG> <STRONG>SPECIFICATION</STRONG>
	  void <STRONG>glStencilOp</STRONG>( GLenum <EM>fail</EM>,
			    GLenum <EM>zfail</EM>,
			    GLenum <EM>zpass</EM> )


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

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

	  <EM>zpass</EM>	 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.	<EM>zpass</EM> accepts the same
		 symbolic constants as <EM>fail</EM>. The initial value is
		 <STRONG>GL_KEEP</STRONG>.

     <STRONG>DESCRIPTION</STRONG>
	  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 value	in the stencil
	  buffer and a reference value.	To enable and disable the
	  test,	call <STRONG>glEnable</STRONG> and <STRONG>glDisable</STRONG> with argument
	  <STRONG>GL_STENCIL_TEST</STRONG>; to control it, call <STRONG>glStencilFunc</STRONG>.

	  <STRONG>glStencilOp</STRONG> 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 <EM>fail</EM> specifies what happens to
	  the stencil buffer contents.	The following six actions are
	  possible.

	  <STRONG>GL_KEEP</STRONG>	  Keeps	the current value.

	  <STRONG>GL_ZERO</STRONG>	  Sets the stencil buffer value	to 0.

	  <STRONG>GL_REPLACE</STRONG>	  Sets the stencil buffer value	to <EM>ref</EM>,	as
			  specified by <STRONG>glStencilFunc</STRONG>.

	  <STRONG>GL_INCR</STRONG>	  Increments the current stencil buffer	value.
			  Clamps to the	maximum	representable unsigned
			  value.

	  <STRONG>GL_DECR</STRONG>	  Decrements the current stencil buffer	value.
			  Clamps to 0.

	  <STRONG>GL_INVERT</STRONG>	  Bitwise inverts the current stencil buffer
			  value.

	  Stencil buffer values	are treated as unsigned	integers.
	  When incremented and decremented, values are clamped to 0
	  and 2n-1, where n is the value returned by querying
	  <STRONG>GL_STENCIL_BITS</STRONG>.

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

     <STRONG>NOTES</STRONG>
	  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 always pass, regardless of any call
	  to <STRONG>glStencilOp</STRONG>.

     <STRONG>ERRORS</STRONG>
	  <STRONG>GL_INVALID_ENUM</STRONG> is generated if <EM>fail</EM>,	<EM>zfail</EM>, or <EM>zpass</EM>	is any
	  value	other than the six defined constant values.

	  <STRONG>GL_INVALID_OPERATION</STRONG> is generated if <STRONG>glStencilOp</STRONG> is executed
	  between the execution	of <STRONG>glBegin</STRONG> and the corresponding
	  execution of <STRONG>glEnd</STRONG>.

     <STRONG>ASSOCIATED</STRONG>	<STRONG>GETS</STRONG>
	  <STRONG>glGet</STRONG>	with argument <STRONG>GL_STENCIL_FAIL</STRONG>
	  <STRONG>glGet</STRONG>	with argument <STRONG>GL_STENCIL_PASS_DEPTH_PASS</STRONG>
	  <STRONG>glGet</STRONG>	with argument <STRONG>GL_STENCIL_PASS_DEPTH_FAIL</STRONG>
	  <STRONG>glGet</STRONG>	with argument <STRONG>GL_STENCIL_BITS</STRONG>
	  <STRONG>glIsEnabled</STRONG> with argument <STRONG>GL_STENCIL_TEST</STRONG>

     <STRONG>SEE</STRONG> <STRONG>ALSO</STRONG>
	  <STRONG>glAlphaFunc</STRONG>, <STRONG>glBlendFunc</STRONG>, <STRONG>glDepthFunc</STRONG>, <STRONG>glEnable</STRONG>, <STRONG>glLogicOp</STRONG>,
	  <STRONG>glStencilFunc</STRONG>



















































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