GLSTENCILFUNC(3G) UNIX Programmer's Manual GLSTENCILFUNC(3G)
NAME
glStencilFunc - set function and reference value for stencil
testing
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 [0,2n-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 draw-
ing 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, out-
lining, 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 com-
parison 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 [0,2n-1], where n is the number of bitplanes in the
stencil buffer. mask is bitwise ANDed with both the refer-
ence value and the stored stencil value, with the ANDed
values participating in the comparison.
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GLSTENCILFUNC(3G) UNIX Programmer's Manual GLSTENCILFUNC(3G)
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 [0,2n-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 ).
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 exe-
cuted 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
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GLSTENCILFUNC(3G) UNIX Programmer's Manual GLSTENCILFUNC(3G)
SEE ALSO
glAlphaFunc(3G), glBlendFunc(3G), glDepthFunc(3G),
glEnable(3G), glIsEnabled(3G), glLogicOp(3G),
glStencilOp(3G)
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