NAME
	  fglStencilOp - set stencil test actions


     FORTRAN SPECIFICATION
	  SUBROUTINE fglStencilOp( INTEGER*4 fail,
				   INTEGER*4 zfail,
				   INTEGER*4 zpass )


     PARAMETERS
	  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.

     DESCRIPTION
	  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 fglEnable and fglDisable with argument
	  GL_STENCIL_TEST; to control it, call fglStencilFunc.

	  fglStencilOp 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 following 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 fglStencilFunc.

	  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 0.

	  GL_INVERT	  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
	  GL_STENCIL_BITS.

	  The other two	arguments to fglStencilOp specify stencil
	  buffer actions that depend on	whether	subsequent depth
	  buffer tests succeed (zpass) or fail (zfail) (see
	  fglDepthFunc).  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.

     NOTES
	  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 fglStencilOp.

     ERRORS
	  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 fglStencilOp is
	  executed between the execution of fglBegin and the
	  corresponding	execution of fglEnd.

     ASSOCIATED	GETS
	  fglGet with argument GL_STENCIL_FAIL
	  fglGet with argument GL_STENCIL_PASS_DEPTH_PASS
	  fglGet with argument GL_STENCIL_PASS_DEPTH_FAIL
	  fglGet with argument GL_STENCIL_BITS
	  fglIsEnabled with argument GL_STENCIL_TEST

     SEE ALSO
	  fglAlphaFunc,	fglBlendFunc, fglDepthFunc, fglEnable,
	  fglLogicOp, fglStencilFunc