fglStencilFunc(3G) OpenGL Reference 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.
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fglStencilFunc(3G) OpenGL Reference fglStencilFunc(3G)
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
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