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glStencilOp
Chapter 17
406
glStencilOp
glStencilOp: set stencil test actions.
C Specification
void glStencilOp(
GLenum fail,
GLenum zfail,
GLenum zpass)
Parameters
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.
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 planesto 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.
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.
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