GLSTENCILOP(3G) GLSTENCILOP(3G)
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, 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 algo-rithms algorithms
rithms to achieve special effects, such as decals, outlining, and constructive solid geometry render-ing. rendering.
ing.
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 sten-ciling stenciling
ciling 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 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_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^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 subse-quent subsequent
quent 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 con-stant constant
stant 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(3G), glBlendFunc(3G), glDepthFunc(3G), glEnable(3G), glLogicOp(3G), glStencilFunc(3G)
GLSTENCILOP(3G)
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