Signature
Parameters
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
GL_STENCIL_TEST
; to control it, call
glStencilFunc
or
glStencilFuncSeparate
.
There can be two separate sets of
sfail
,
dpfail
, and
dppass
parameters; one affects back-facing polygons, and the other
affects front-facing polygons as well as other non-polygon primitives.
glStencilOp
sets both front
and back stencil state to the same values. Use
glStencilOpSeparate
to set front and back stencil state to different values.
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
sfail
specifies what happens to the stencil buffer contents.
The following eight actions are possible.
- GL_KEEP
-
Keeps the current value.
- GL_ZERO
-
Sets the stencil buffer value to 0.
- 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_INCR_WRAP
-
Increments the current stencil buffer value. Wraps stencil buffer value to zero when incrementing the maximum representable unsigned value.
- GL_DECR
-
Decrements the current stencil buffer value. Clamps to 0.
- GL_DECR_WRAP
-
Decrements the current stencil buffer value. Wraps stencil buffer value to the maximum representable unsigned value when decrementing a stencil buffer value of 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
,
where
is the value returned by querying
GL_STENCIL_BITS
.
The other two arguments to
glStencilOp
specify stencil buffer actions
that depend on whether subsequent depth buffer tests succeed (
dppass
)
or fail (
dpfail
) (see
glDepthFunc
).
The actions are specified using the same eight symbolic constants as
sfail
.
Note that
dpfail
is ignored when there is no depth buffer,
or when the depth buffer is not enabled.
In these cases,
sfail
and
dppass
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
Associated Gets
glGet
with argument
GL_STENCIL_FAIL
,
GL_STENCIL_PASS_DEPTH_PASS
,
GL_STENCIL_PASS_DEPTH_FAIL
,
GL_STENCIL_BACK_FAIL
,
GL_STENCIL_BACK_PASS_DEPTH_PASS
,
GL_STENCIL_BACK_PASS_DEPTH_FAIL
,
or
GL_STENCIL_BITS
glIsEnabled
with argument
GL_STENCIL_TEST
Version Support
| glStencilOp |
See Also
Copyright
Copyright
1991-2006 Silicon Graphics, Inc.
Copyright
2010-2014 Khronos Group.
This document is licensed under the SGI Free Software B License.
For details, see
http://oss.sgi.com/projects/FreeB/
.
Sample Code References
The following code samples have been found which appear to reference the functions described here. Take care that the code may be old, broken or not even use PyOpenGL.
glStencilOp
