GLSTENCILOP(3gl)GLSTENCILOP(3gl)NAMEglStencilOp - 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_INCR_WRAP, GL_DECR, GL_DECR_WRAP, 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 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 fol‐
lowing 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 representable unsigned value.
GL_INCR_WRAP Increments the current stencil buffer value. Incre‐
menting maximum representable unsigned value will wrap
to 0.
GL_DECR Decrements the current stencil buffer value. Clamps to
0.
GL_DECR_WRAP Decrements the current stencil buffer value. Decre‐
menting 0 will wrap to maximum representable unsigned
value.
GL_INVERT 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 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.
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, glStencil‐
Func
15 Mar 97 GLSTENCILOP(3gl)
