glStencilOp — set front and back stencil test actions
void glStencilOp( | GLenum | sfail, |
| GLenum | dpfail, | |
| GLenum | dppass); |
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 GL_KEEP.
dpfail
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.
dppass
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 GL_KEEP.
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 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.
GL_KEEPKeeps the current value.
GL_ZEROSets the stencil buffer value to 0.
GL_REPLACESets the stencil buffer value to ref, as specified by glStencilFunc.
GL_INCRIncrements the current stencil buffer value. Clamps to the maximum representable unsigned value.
GL_INCR_WRAPIncrements the current stencil buffer value. Wraps stencil buffer value to zero when incrementing the maximum representable unsigned value.
GL_DECRDecrements the current stencil buffer value. Clamps to 0.
GL_DECR_WRAPDecrements the current stencil buffer value. Wraps stencil buffer value to the maximum representable unsigned value when decrementing a stencil buffer value of zero.
GL_INVERTBitwise inverts the current stencil buffer value.
Stencil buffer values are treated as unsigned integers.
When incremented and decremented,
values are clamped to 0 and
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_FRONT_AND_BACK.
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_PASS_DEPTH_PASS,
GL_STENCIL_PASS_DEPTH_FAIL, GL_STENCIL_BACK_FAIL,
GL_STENCIL_BACK_PASS_DEPTH_PASS, GL_STENCIL_BACK_PASS_DEPTH_FAIL,
or GL_STENCIL_BITS
glIsEnabled with argument GL_STENCIL_TEST
glBlendFunc, glDepthFunc, glEnable, glLogicOp, glStencilFunc, glStencilFuncSeparate, glStencilMask, glStencilMaskSeparate, glStencilOpSeparate
Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.