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_C_ Original Code. The Original Code is: OpenGL Sample Implementation,
_C_ Version 1.2.1, released January 26, 2000, developed by Silicon Graphics,
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_C_ Additional Notice Provisions: The application programming interfaces
_C_ established by SGI in conjunction with the Original Code are The
_C_ OpenGL(R) Graphics System: A Specification (Version 1.2.1), released
_C_ April 1, 1999; The OpenGL(R) Graphics System Utility Library (Version
_C_ 1.3), released November 4, 1998; and OpenGL(R) Graphics with the X
_C_ Window System(R) (Version 1.3), released October 19, 1998. This software
_C_ was created using the OpenGL(R) version 1.2.1 Sample Implementation
_C_ published by SGI, but has not been independently verified as being
_C_ compliant with the OpenGL(R) version 1.2.1 Specification.
_C_
_C_ The first character in this file must be an '_'!
_C_ Anything on a line after _C_ is ignored
_define(_filters,tbl|eqn)_C_
_C_      eqn is automatically replaced with neqn for nroff
_header(StencilOp,set stencil test actions)
.EQ
delim $$
.EN
_names(StencilOp)
.SH PARAMETERS
_phead(_param1)
Specifies the action to take when the stencil test fails.
Six symbolic constants are accepted:
_const(KEEP),
_const(ZERO),
_const(REPLACE),
_const(INCR),
_const(DECR), and
_const(INVERT). The initial value is _const(KEEP). 
_phead(_param2)
Specifies the stencil action when the stencil test passes,
but the depth test fails.
_param2 accepts the same symbolic constants as _param1. The initial value
is _const(KEEP).
_phead(_param3)
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.
_param3 accepts the same symbolic constants as _param1. The initial value
is _const(KEEP).
.SH 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.
.P
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 _cmnd(Enable)
and _cmnd(Disable) with argument
_const(STENCIL_TEST); to control it, call _cmnd(StencilFunc).
.P
_cmnd 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 _param1 specifies what happens to the stencil buffer contents.
The following six actions are possible.
.TP 16
_const( KEEP)
Keeps the current value.
.TP
_const( ZERO)
Sets the stencil buffer value to 0.
.TP
_const( REPLACE)
Sets the stencil buffer value to \f2ref\fP,
as specified by _cmnd(StencilFunc).
.TP
_const( INCR)
Increments the current stencil buffer value.
Clamps to the maximum representable unsigned value.
.TP
_const( DECR)
Decrements the current stencil buffer value.
Clamps to 0.
.TP
_const( INVERT)
Bitwise inverts the current stencil buffer value.
.P
Stencil buffer values are treated as unsigned integers.
When incremented and decremented,
values are clamped to 0 and $2 sup n - 1$,
where $n$ is the value returned by querying _const(STENCIL_BITS).
.P
The other two arguments to _cmnd specify stencil buffer actions
that depend on whether subsequent depth buffer tests succeed (_param3)
or fail (_param2) (see 
.br
_cmnd(DepthFunc)).
The actions are specified using the same six symbolic constants as _param1.
Note that _param2 is ignored when there is no depth buffer,
or when the depth buffer is not enabled.
In these cases, _param1 and _param3 specify stencil action when the
stencil test fails and passes,
respectively.
.SH 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 _cmnd.
.SH ERRORS
_const(INVALID_ENUM) is generated if _param1,
_param2, or _param3 is any value other than the six defined constant values.
.P
_const(INVALID_OPERATION) is generated if _cmnd
is executed between the execution of _cmnd(Begin)
and the corresponding execution of _cmnd(End).
.SH ASSOCIATED GETS
_cmnd(Get) with argument _const(STENCIL_FAIL)
.br
_cmnd(Get) with argument _const(STENCIL_PASS_DEPTH_PASS)
.br
_cmnd(Get) with argument _const(STENCIL_PASS_DEPTH_FAIL)
.br
_cmnd(Get) with argument _const(STENCIL_BITS)
.br
_cmnd(IsEnabled) with argument _const(STENCIL_TEST)
.SH SEE ALSO
_cmnd(AlphaFunc),
_cmnd(BlendFunc),
_cmnd(DepthFunc),
_cmnd(Enable),
_cmnd(LogicOp),
_cmnd(StencilFunc)