GLSTENCILFUNC(3G) 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 [0,2^n - 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 algo-rithms algorithms
rithms to achieve special effects, such as decals, outlining, and constructive solid geometry render-ing. rendering.
ing.
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 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 [0,2^n - 1], where n is the number of bitplanes in the sten-cil stencil
cil 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 compar-ison comparison
ison 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 [0,2^n - 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 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(3G), glBlendFunc(3G), glDepthFunc(3G), glEnable(3G), glIsEnabled(3G), glLogicOp(3G), glStencilOp(3G)
GLSTENCILFUNC(3G)
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