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 ac‐
              cepts 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  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 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  following
       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.

       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 al‐
       ways 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)