- set stencil test actions
( GLenum fail
GLenum zpass )
- 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. The initial value is
- 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.
- 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.
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
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
; to control it, call glStencilFunc
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.
- Keeps the current value.
- Sets the stencil buffer value to 0.
- Sets the stencil buffer value to ref, as specified by
- Increments the current stencil buffer value. Clamps to the maximum
representable unsigned value.
- Decrements the current stencil buffer value. Clamps to 0.
- 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
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
). 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,
specify stencil action when the stencil test
fails and passes, respectively.
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
is generated if fail
is any value other than the six defined constant values.
is generated if glStencilOp
between the execution of glBegin
and the corresponding execution of
with argument GL_STENCIL_FAIL
with argument GL_STENCIL_PASS_DEPTH_PASS
with argument GL_STENCIL_PASS_DEPTH_FAIL
with argument GL_STENCIL_BITS
with argument GL_STENCIL_TEST