glStencilOp(3G)glStencilOp(3G)NAMEglStencilOp - set stencil test actions
SYNOPSIS
void glStencilOp(
GLenum fail,
GLenum zfail,
GLenum zpass );
PARAMETERS
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 GL_KEEP. 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.
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().
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 follow‐
ing six actions are possible. Keeps the current value. Sets the sten‐
cil buffer value to 0. Sets the stencil buffer value to ref, as speci‐
fied by glStencilFunc(). 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 incre‐
mented and decremented, values are clamped to 0 and 2 sup n - 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().
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 GETSglGet() 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 ALSOglAlphaFunc(3), glBlendFunc(3), glDepthFunc(3), glEnable(3), glLogi‐
cOp(3), glStencilFunc(3)glStencilOp(3G)