glStencilFunc man page on Darwin

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GLSTENCILFUNC(3G)					     GLSTENCILFUNC(3G)

NAME
       glStencilFunc - set function and reference value for stencil testing

C SPECIFICATION
       void glStencilFunc( GLenum func,
			   GLint ref,
			   GLuint mask )

PARAMETERS
       func  Specifies	the  test function.  Eight tokens are valid: GL_NEVER,
	     GL_LESS, GL_LEQUAL, GL_GREATER, GL_GEQUAL, GL_EQUAL, GL_NOTEQUAL,
	     and GL_ALWAYS. The initial value is GL_ALWAYS.

       ref   Specifies	the  reference	value  for  the	 stencil test.	ref is
	     clamped to the range [0,2^n - 1], where n is the number  of  bit‐
	     planes in the stencil buffer. The initial value is 0.

       mask  Specifies	a mask that is ANDed with both the reference value and
	     the stored stencil value when the test is done. The initial value
	     is all 1's.

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 reference value and the value in the sten‐
       cil buffer.   To	 enable	 and  disable  the  test,  call	 glEnable  and
       glDisable  with	argument GL_STENCIL_TEST.  To specify actions based on
       the outcome of the stencil test, call glStencilOp.

       func is a symbolic constant  that  determines  the  stencil  comparison
       function.  It accepts one of eight values, shown in the following list.
       ref is an integer reference value that is used in the stencil  compari‐
       son.   It is clamped to the range [0,2^n - 1], where n is the number of
       bitplanes in the stencil buffer.	 mask is bitwise ANDed with  both  the
       reference  value	 and  the  stored stencil value, with the ANDed values
       participating in the comparison.

       If stencil represents the value stored  in  the	corresponding  stencil
       buffer location, the following list shows the effect of each comparison
       function that can be specified by func.	Only if	 the  comparison  suc‐
       ceeds  is  the pixel passed through to the next stage in the rasteriza‐
       tion process (see glStencilOp).	All  tests  treat  stencil  values  as
       unsigned	 integers  in  the range [0,2^n - 1], where n is the number of
       bitplanes in the stencil buffer.

       The following values are accepted by func:

       GL_NEVER		 Always fails.

       GL_LESS		 Passes if ( ref & mask ) < ( stencil & mask ).

       GL_LEQUAL	 Passes if ( ref & mask ) ≤ ( stencil & mask ).

       GL_GREATER	 Passes if ( ref & mask ) > ( stencil & mask ).

       GL_GEQUAL	 Passes if ( ref & mask ) ≥ ( stencil & mask ).

       GL_EQUAL		 Passes if ( ref & mask ) = ( stencil & mask ).

       GL_NOTEQUAL	 Passes if ( ref & mask ) ≠  ( stencil & mask ).

       GL_ALWAYS	 Always passes.

NOTES
       Initially, the stencil test is disabled.	 If there is no	 stencil  buf‐
       fer, no stencil modification can occur and it is as if the stencil test
       always passes.

ERRORS
       GL_INVALID_ENUM is generated if func is not one of the  eight  accepted
       values.

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

ASSOCIATED GETS
       glGet with argument GL_STENCIL_FUNC
       glGet with argument GL_STENCIL_VALUE_MASK
       glGet with argument GL_STENCIL_REF
       glGet with argument GL_STENCIL_BITS
       glIsEnabled with argument GL_STENCIL_TEST

SEE ALSO
       glAlphaFunc,   glBlendFunc,   glDepthFunc,    glEnable,	  glIsEnabled,
       glLogicOp, glStencilOp

							     GLSTENCILFUNC(3G)
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