glStencilOp — set stencil test actions
void fsfuncglStencilOp( | GLenum fail, |
| GLenum zfail, | |
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. 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.
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 stencil test, call
glEnable and
glDisable
with argument GL_STENCIL_TEST.
To control it, call
glStencilFunc.
Stenciling is initially disabled.
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
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
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 (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.
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.
Copyright © 2003-2004 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.