glStencilOp - set front and back stencil test actions
void glStencilOp(GLenum sfail,
Specifies the action to take when the stencil test fails.
Eight 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
Specifies the stencil action when the stencil test
passes, but the depth test fails. dpfail accepts the same symbolic
constants as sfail. 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. dppass accepts the
same symbolic constants as sfail. The initial value is
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
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()
glDisable with argument GL_STENCIL_TEST; to control
it, call glStencilFunc() or glStencilFuncSeparate().
There can be two separate sets of sfail, dpfail, and
dppass parameters; one affects back-facing polygons, and the other
affects front-facing polygons as well as other non-polygon primitives.
glStencilOp() sets both front and back stencil state to the same
values. Use glStencilOpSeparate() to set front and back stencil state
to different values.
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 sfail specifies what happens to the stencil buffer contents. The
following eight actions are possible.
Keeps the current value.
Sets the stencil buffer value to 0.
Sets the stencil buffer value to ref, as specified
Increments the current stencil buffer value. Clamps to
the maximum representable unsigned value.
Increments the current stencil buffer value. Wraps
stencil buffer value to zero when incrementing the maximum representable
Decrements the current stencil buffer value. Clamps to
Decrements the current stencil buffer value. Wraps
stencil buffer value to the maximum representable unsigned value when
decrementing a stencil buffer value of zero.
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 subsequent depth buffer tests succeed
(dppass) or fail (dpfail) (see glDepthFunc()). The
actions are specified using the same eight symbolic constants as
sfail. Note that dpfail is ignored when there is no depth
buffer, or when the depth buffer is not enabled. In these cases,
sfail and dppass 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.
glStencilOp() is the same as calling
glStencilOpSeparate() with face set to
GL_INVALID_ENUM is generated if sfail, dpfail, or
dppass is any value other than the defined constant values.
glGet() with argument GL_STENCIL_FAIL,
GL_STENCIL_BACK_PASS_DEPTH_FAIL, or GL_STENCIL_BITS
glIsEnabled() with argument GL_STENCIL_TEST
glBlendFunc(), glDepthFunc(), glEnable(),
glLogicOp(), glStencilFunc(), glStencilFuncSeparate(),
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Khronos Group. This document is licensed under the SGI Free Software B
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Copyright © 1991-2006 Silicon Graphics, Inc.
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