GLSTENCILOP() MachTen Programmer’s Manual GLSTENCILOP()

NAME
glStencilOp - set stencil test actions

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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. 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. 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 render-
ing.

The stencil test conditionally eliminates a pixel based on
the outcome of a comparison between the value in the sten-
cil 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 hap-
pens 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.

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 repre-
sentable unsigned value.

GL_DECR Decrements the current stencil buffer
value. Clamps to 0.

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 $2 sup n - 1$, 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 exe-
cuted between the execution of glBegin and the correspond-
ing 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, glLogi-
cOp, glStencilFunc

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