glStencilOp man page on Darwin

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GLSTENCILOP(3G)						       GLSTENCILOP(3G)

NAME
       glStencilOp - set stencil test actions

C SPECIFICATION
       void glStencilOp( GLenum fail,
			 GLenum zfail,
			 GLenum 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 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
		       glStencilFunc.

       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	incre‐
       mented and decremented, values are clamped to 0 and 2^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.

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 glStencilOp.

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

ASSOCIATED GETS
       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

SEE ALSO
       glAlphaFunc,    glBlendFunc,    glDepthFunc,    glEnable,    glLogicOp,
       glStencilFunc

							       GLSTENCILOP(3G)
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