OpenGL man pages
glStencilOp - set stencil test actions
void glStencilOp( GLenum fail,
GLenum zpass )
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.
zfail Specifies stencil action when the stencil test passes, but the depth
test fails. zfail accepts the same symbolic constants as fail.
zpass Specifies 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.
Stenciling, like z-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.
The test is enabled with glEnable and glDisable calls with argument
GL_STENCIL_TEST, and controlled with 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 six possible actions are
GL_KEEP Keeps the current value.
GL_ZERO Sets the stencil buffer value to zero.
GL_REPLACE Sets the stencil buffer value to ref, as specified by
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
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 2n-1, where n is the value
returned by querying GL_STENCIL_BITS.
The other two arguments to glStencilOp specify stencil buffer actions
should subsequent depth buffer tests succeed (zpass) or fail (zfail). (See
glDepthFunc.) They 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.
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.
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.
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
glAlphaFunc, glBlendFunc, glDepthFunc, glEnable, glLogicOp, glStencilFunc
Fri Dec 6 11:18:03 EST 1996
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