fglStencilFunc(3G)

NAME
     fglStencilFunc - set function and reference value for stencil testing


FORTRAN SPECIFICATION
     SUBROUTINE fglStencilFunc( INTEGER*4 func,
                                INTEGER*4 ref,
                                INTEGER*4 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
                            n
           to the range [0,2 -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 fglEnable and fglDisable
     with argument GL_STENCIL_TEST.  To specify actions based on the outcome
     of the stencil test, call fglStencilOp.

     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.
                                    n
     It is clamped to the range [0,2 -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 fglStencilOp).  All tests treat stencil values as unsigned
                               n
     integers in the range [0,2 -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 fglStencilFunc is executed between
     the execution of fglBegin and the corresponding execution of fglEnd.

ASSOCIATED GETS
     fglGet with argument GL_STENCIL_FUNC
     fglGet with argument GL_STENCIL_VALUE_MASK
     fglGet with argument GL_STENCIL_REF
     fglGet with argument GL_STENCIL_BITS
     fglIsEnabled with argument GL_STENCIL_TEST


SEE ALSO
     fglAlphaFunc, fglBlendFunc, fglDepthFunc, fglEnable, fglIsEnabled,
     fglLogicOp, fglStencilOp