glStencilOp man page on RedHat
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GLSTENCILOP(3G) OpenGL Manual GLSTENCILOP(3G)
NAME
glStencilOp - set front and back stencil test actions
C SPECIFICATION
void glStencilOp(GLenum sfail, GLenum dpfail, GLenum dppass);
PARAMETERS
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.
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() 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_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. Wraps stencil buffer
value to zero when incrementing the maximum representable unsigned
value.
GL_DECR
Decrements the current stencil buffer value. Clamps to 0.
GL_DECR_WRAP
Decrements the current stencil buffer value. Wraps stencil buffer
value to the maximum representable unsigned value when decrementing
a stencil buffer value of zero.
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 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.
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.
glStencilOp() is the same as calling glStencilOpSeparate() with face
set to GL_FRONT_AND_BACK.
ERRORS
GL_INVALID_ENUM is generated if sfail, dpfail, or dppass is any value
other than the defined constant values.
ASSOCIATED GETS
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
SEE ALSO
glBlendFunc(), glDepthFunc(), glEnable(), glLogicOp(), glStencilFunc(),
glStencilFuncSeparate(), glStencilMask(), glStencilMaskSeparate(),
glStencilOpSeparate()
COPYRIGHT
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/.
AUTHORS
opengl.org
opengl.org 03/16/2013 GLSTENCILOP(3G)
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