glStencilOp(3G) OpenGL Reference glStencilOp(3G)
NAME
glStencilOp - set stencil test actions
C SPECIFICATION
void glStencilOp( GLenum fail,
GLenum zfail,
GLenum zpass )
PARAMETERS
fail Specifies the action to take when the stencil test fails. Six
symbolic constants are accepted: GLKEEP, GLZERO, GLREPLACE,
GLINCR, GLDECR, and GLINVERT. The initial value is GLKEEP.
zfail Specifies the stencil action when the stencil test passes, but the
depth test fails. zfail accepts the same symbolic constants as
fail. The initial value is GLKEEP.
zpass Specifies the stencil action when both the stencil test and the
depth test pass, or when the stencil test passes and either there
is no depth buffer or depth testing is not enabled. zpass accepts
the same symbolic constants as fail. The initial value is GLKEEP.
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 value in the stencil buffer and a reference
value. To enable and disable the test, call glEnable and glDisable with
argument GLSTENCILTEST; to control it, call glStencilFunc.
glStencilOp takes three arguments that indicate what happens to the
stored stencil value while stenciling is enabled. If the stencil test
fails, no change is made to the pixel's color or depth buffers, and fail
specifies what happens to the stencil buffer contents. The following six
actions are possible.
GLKEEP Keeps the current value.
GLZERO Sets the stencil buffer value to 0.
GLREPLACE Sets the stencil buffer value to ref, as specified by
glStencilFunc.
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glStencilOp(3G) OpenGL Reference glStencilOp(3G)
GLINCR Increments the current stencil buffer value. Clamps to
the maximum representable unsigned value.
GLDECR Decrements the current stencil buffer value. Clamps to
0.
GLINVERT Bitwise inverts the current stencil buffer value.
Stencil buffer values are treated as unsigned integers. When incremented
n
and decremented, values are clamped to 0 and 2 -1, where n is the value
returned by querying GLSTENCILBITS.
The other two arguments to glStencilOp specify stencil buffer actions
that depend on whether subsequent depth buffer tests succeed (zpass) or
fail (zfail) (see
glDepthFunc). The actions are specified using the same six symbolic
constants as fail. Note that zfail is ignored when there is no depth
buffer, or when the depth buffer is not enabled. In these cases, fail
and zpass specify stencil action when the stencil test fails and passes,
respectively.
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 tests
always pass, regardless of any call to glStencilOp.
ERRORS
GLINVALIDENUM is generated if fail, zfail, or zpass is any value other
than the six defined constant values.
GLINVALIDOPERATION is generated if glStencilOp is executed between the
execution of glBegin and the corresponding execution of glEnd.
ASSOCIATED GETS
glGet with argument GLSTENCILFAIL
glGet with argument GLSTENCILPASSDEPTHPASS
glGet with argument GLSTENCILPASSDEPTHFAIL
glGet with argument GLSTENCILBITS
glIsEnabled with argument GLSTENCILTEST
SEE ALSO
glAlphaFunc, glBlendFunc, glDepthFunc, glEnable, glLogicOp, glStencilFunc
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