glStencilFunc(3) glStencilFunc(3)
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 bit-
planes 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 sten-
cil 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 compari-
son. It is clamped to the range [0,2^n - 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 suc-
ceeds is the pixel passed through to the next stage in the rasteriza-
tion 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(3), glBlendFunc(3), glDepthFunc(3), glEnable(3),
glIsEnabled(3), glLogicOp(3), glStencilOp(3)
glStencilFunc(3)
Mac OS X 10.6 - Generated Thu Sep 17 20:20:29 CDT 2009