glStencilOp
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: GL_KEEP, GL_ZERO,
GL_REPLACE, GL_INCR, GL_DECR, and
GL_INVERT.
- zfail
-
Specifies stencil action when the stencil test passes, but the depth
test fails. zfail accepts the same symbolic constants as
fail.
- zpass
-
Specifies 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.
DESCRIPTION
Stenciling, like z-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. The test is enabled with
glEnable and
glDisable
calls with argument GL_STENCIL_TEST, and controlled with
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 six possible actions are as follows:
- GL_KEEP
-
Keeps the current value.
- GL_ZERO
-
Sets the stencil buffer value to zero.
- GL_REPLACE
-
Sets the stencil buffer value to ref, as specified by
glStencilFunc.
- GL_INCR
-
Increments the current stencil buffer value. Clamps to the
maximum representable unsigned value.
- GL_DECR
-
Decrements the current stencil buffer value. Clamps to zero.
- GL_INVERT
-
Bitwise inverts the current stencil buffer value.
Stencil buffer values are treated as unsigned integers. When incremented
and decremented, values are clamped to 0 and 2^n - 1, where n
is the value returned by querying GL_STENCIL_BITS.
The other two arguments to glStencilOp specify stencil buffer actions
should subsequent depth buffer tests succeed (zpass) or fail
(zfail). (See
glDepthFunc.)
They 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
GL_INVALID_ENUM is generated if fail, zfail, or
zpass is any value other than the six defined constant values.
GL_INVALID_OPERATION is generated if glStencilOp is called
between a call to glBegin and the
corresponding call to glEnd.
ASSOCIATED GETS
glGet with argument
GL_STENCIL_FAIL
glGet with argument
GL_STENCIL_PASS_DEPTH_PASS
glGet with argument
GL_STENCIL_PASS_DEPTH_FAIL
glGet with argument
GL_STENCIL_BITS
glIsEnabled with argument
GL_STENCIL_TEST
SEE ALSO
glAlphaFunc,
glBlendFunc,
glDepthFunc,
glEnable,
glLogicOp,
glStencilFunc
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© 1995 Uwe Behrens. All rights reserved.