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, GL_INVERT, GL_INCR_WRAP_EXT,
and GL_DECR_WRAP_EXT. The initial value is GL_KEEP.
|
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 GL_KEEP.
|
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 GL_KEEP.
|
DESCRIPTION
Stenciling, like depth-buffering, enables and disables
drawing on a per-pixel basis. GL drawing primitives are
used to draw into the stencil planes which can then be
used to mask out portions of the screen when rendering
geometry and images. 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
glDisable with
argument GL_STENCIL_TEST; 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.
- 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.
- GL_INCR_WRAP_EXT
-
Increments the current stencil buffer value such that incrementing
the maximum representable value results in a value of 0.
- GL_DECR_WRAP_EXT
-
Decrements the current stencil buffer value such that decrementing
the a value of 0 results in the maximum representable value.
Stencil buffer values are treated as unsigned integers.
When incremented and decremented, values are clamped to 0
and 2n - 1, where n 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 (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.
EXTENSIONS
Some routines and constants for this function are part of an extension, not part of the
core GL command set. The following extension names must be present in the string returned by
glGetString when called with argument GL_EXTENSIONS
to use these routines and constants.
- GL_EXT_stencil_wrap
-
GL_INCR_WRAP_EXT and GL_DECR_WRAP_EXT
are part of the EXT_stencil_wrap extension.
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 executed
between the execution of glBegin and the
corresponding execution of 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