glStencilOp

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 con-
	      stants 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  refer-
       ence  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 2n−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(3G),	  glBlendFunc(3G),   glDepthFunc(3G),	glEnable(3G),
       glLogicOp(3G), glStencilFunc(3G)



							      GLSTENCILOP(3G)