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 [0,2n−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  primi‐
       tives, 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, outlin‐
       ing, 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 func‐
       tion.  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 [0,2n−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 buf‐
       fer  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 [0,2n−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 ).

       no  stencil  modification can occur and it is as if the stencil test al‐
       ways 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)