glStencilFunc man page on OpenBSD

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GLSTENCILFUNC(3G)					     GLSTENCILFUNC(3G)

NAME
       glStencilFunc - set function and reference value for stencil testing

       delim $$

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,$2 sup n - 1$], where $n$ is the number
	      of bitplanes 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
       stencil 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
       comparison.  It is clamped to the range [0,$2 sup n - 1$], 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
       succeeds is the pixel passed through to the next stage in the
       rasterization process (see glStencilOp).	 All tests treat stencil
       values as unsigned integers in the range [0,$2 sup n - 1$], 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
       buffer, 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(3G), glBlendFunc(3G), glDepthFunc(3G), glEnable(3G),
       glIsEnabled(3G), glLogicOp(3G), glStencilOp(3G)

								 March 1, 2011
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