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< !DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
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<refentry id= "glStencilFunc" >
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<refentryinfo >
<copyright >
<year > 1991-2006</year>
<holder > Silicon Graphics, Inc.</holder>
</copyright>
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<copyright >
<year > 2010-2013</year>
<holder > Khronos Group</holder>
</copyright>
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</refentryinfo>
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<refmeta >
<refentrytitle > glStencilFunc</refentrytitle>
<manvolnum > 3G</manvolnum>
</refmeta>
<refnamediv >
<refname > glStencilFunc</refname>
<refpurpose > set front and back function and reference value for stencil testing</refpurpose>
</refnamediv>
<refsynopsisdiv > <title > C Specification</title>
<funcsynopsis >
<funcprototype >
<funcdef > void <function > glStencilFunc</function> </funcdef>
<paramdef > GLenum <parameter > func</parameter> </paramdef>
<paramdef > GLint <parameter > ref</parameter> </paramdef>
<paramdef > GLuint <parameter > mask</parameter> </paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1 id= "parameters" > <title > Parameters</title>
<variablelist >
<varlistentry >
<term > <parameter > func</parameter> </term>
<listitem >
<para >
Specifies the test function.
Eight symbolic constants are valid:
<constant > GL_NEVER</constant> ,
<constant > GL_LESS</constant> ,
<constant > GL_LEQUAL</constant> ,
<constant > GL_GREATER</constant> ,
<constant > GL_GEQUAL</constant> ,
<constant > GL_EQUAL</constant> ,
<constant > GL_NOTEQUAL</constant> , and
<constant > GL_ALWAYS</constant> . The initial value is <constant > GL_ALWAYS</constant> .
</para>
</listitem>
</varlistentry>
<varlistentry >
<term > <parameter > ref</parameter> </term>
<listitem >
<para >
Specifies the reference value for the stencil test.
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<parameter > ref</parameter> is clamped to the range
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<inlineequation > <mml:math >
<!-- eqn: [0, 2 sup n - 1]: -->
<mml:mfenced open= "[" close= "]" >
<mml:mn > 0</mml:mn>
<mml:mrow >
<mml:msup > <mml:mn > 2</mml:mn>
<mml:mi mathvariant= "italic" > n</mml:mi>
</mml:msup>
<mml:mo > -</mml:mo>
<mml:mn > 1</mml:mn>
</mml:mrow>
</mml:mfenced>
</mml:math> </inlineequation> ,
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where
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<inlineequation > <mml:math > <mml:mi mathvariant= "italic" > n</mml:mi> </mml:math> </inlineequation>
is the number of bitplanes in the stencil buffer. The
initial value is 0.
</para>
</listitem>
</varlistentry>
<varlistentry >
<term > <parameter > mask</parameter> </term>
<listitem >
<para >
Specifies a mask that is ANDed with both the reference value
and the stored stencil value when the test is done. The initial value
is all 1's.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id= "description" > <title > Description</title>
<para >
Stenciling,
like depth-buffering,
enables and disables drawing on a per-pixel basis.
Stencil planes are first drawn into using GL drawing primitives, then
geometry and images are rendered 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.
</para>
<para >
The stencil test conditionally eliminates a pixel based on the outcome
of a comparison between the reference value
and the value in the stencil buffer.
To enable and disable the test, call <citerefentry > <refentrytitle > glEnable</refentrytitle> </citerefentry> and <citerefentry > <refentrytitle > glDisable</refentrytitle> </citerefentry>
with argument <constant > GL_STENCIL_TEST</constant> .
To specify actions based on the outcome of the stencil test, call
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<citerefentry > <refentrytitle > glStencilOp</refentrytitle> </citerefentry> or
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<citerefentry > <refentrytitle > glStencilOpSeparate</refentrytitle> </citerefentry> .
</para>
<para >
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There can be two separate sets of <parameter > func</parameter> , <parameter > ref</parameter> , and
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<parameter > mask</parameter> parameters; one affects back-facing polygons, and the other
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affects front-facing polygons as well as other non-polygon primitives.
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<citerefentry > <refentrytitle > glStencilFunc</refentrytitle> </citerefentry> sets both front
and back stencil state to the same values. Use <citerefentry > <refentrytitle > glStencilFuncSeparate</refentrytitle> </citerefentry>
to set front and back stencil state to different values.
</para>
<para >
<parameter > func</parameter> is a symbolic constant that determines the stencil comparison function.
It accepts one of eight values,
shown in the following list.
<parameter > ref</parameter> is an integer reference value that is used in the stencil comparison.
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It is clamped to the range
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<inlineequation > <mml:math >
<!-- eqn: [0, 2 sup n - 1]: -->
<mml:mfenced open= "[" close= "]" >
<mml:mn > 0</mml:mn>
<mml:mrow >
<mml:msup > <mml:mn > 2</mml:mn>
<mml:mi mathvariant= "italic" > n</mml:mi>
</mml:msup>
<mml:mo > -</mml:mo>
<mml:mn > 1</mml:mn>
</mml:mrow>
</mml:mfenced>
</mml:math> </inlineequation> ,
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where
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<inlineequation > <mml:math > <mml:mi mathvariant= "italic" > n</mml:mi> </mml:math> </inlineequation>
is the number of bitplanes in the stencil buffer.
<parameter > mask</parameter> is bitwise ANDed with both the reference value
and the stored stencil value,
with the ANDed values participating in the comparison.
</para>
<para >
If <emphasis > stencil</emphasis> represents the value stored in the corresponding
stencil buffer location,
the following list shows the effect of each comparison function
that can be specified by <parameter > func</parameter> .
Only if the comparison succeeds is the pixel passed through
to the next stage in the rasterization process
(see <citerefentry > <refentrytitle > glStencilOp</refentrytitle> </citerefentry> ).
All tests treat <emphasis > stencil</emphasis> values as unsigned integers in the range
<inlineequation > <mml:math >
<!-- eqn: [0, 2 sup n - 1]: -->
<mml:mfenced open= "[" close= "]" >
<mml:mn > 0</mml:mn>
<mml:mrow >
<mml:msup > <mml:mn > 2</mml:mn>
<mml:mi mathvariant= "italic" > n</mml:mi>
</mml:msup>
<mml:mo > -</mml:mo>
<mml:mn > 1</mml:mn>
</mml:mrow>
</mml:mfenced>
</mml:math> </inlineequation> ,
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where
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<inlineequation > <mml:math > <mml:mi mathvariant= "italic" > n</mml:mi> </mml:math> </inlineequation>
is the number of bitplanes in the stencil buffer.
</para>
<para >
The following values are accepted by <parameter > func</parameter> :
</para>
<variablelist >
<varlistentry >
<term > <constant > GL_NEVER</constant> </term>
<listitem >
<para >
Always fails.
</para>
</listitem>
</varlistentry>
<varlistentry >
<term > <constant > GL_LESS</constant> </term>
<listitem >
<para >
Passes if ( <parameter > ref</parameter> & <parameter > mask</parameter> ) < ( <emphasis > stencil</emphasis> & <parameter > mask</parameter> ).
</para>
</listitem>
</varlistentry>
<varlistentry >
<term > <constant > GL_LEQUAL</constant> </term>
<listitem >
<para >
Passes if ( <parameter > ref</parameter> & <parameter > mask</parameter> ) < = ( <emphasis > stencil</emphasis> & <parameter > mask</parameter> ).
</para>
</listitem>
</varlistentry>
<varlistentry >
<term > <constant > GL_GREATER</constant> </term>
<listitem >
<para >
Passes if ( <parameter > ref</parameter> & <parameter > mask</parameter> ) > ( <emphasis > stencil</emphasis> & <parameter > mask</parameter> ).
</para>
</listitem>
</varlistentry>
<varlistentry >
<term > <constant > GL_GEQUAL</constant> </term>
<listitem >
<para >
Passes if ( <parameter > ref</parameter> & <parameter > mask</parameter> ) > = ( <emphasis > stencil</emphasis> & <parameter > mask</parameter> ).
</para>
</listitem>
</varlistentry>
<varlistentry >
<term > <constant > GL_EQUAL</constant> </term>
<listitem >
<para >
Passes if ( <parameter > ref</parameter> & <parameter > mask</parameter> ) = ( <emphasis > stencil</emphasis> & <parameter > mask</parameter> ).
</para>
</listitem>
</varlistentry>
<varlistentry >
<term > <constant > GL_NOTEQUAL</constant> </term>
<listitem >
<para >
Passes if ( <parameter > ref</parameter> & <parameter > mask</parameter> ) != ( <emphasis > stencil</emphasis> & <parameter > mask</parameter> ).
</para>
</listitem>
</varlistentry>
<varlistentry >
<term > <constant > GL_ALWAYS</constant> </term>
<listitem >
<para >
Always passes.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id= "notes" > <title > Notes</title>
<para >
Initially, the stencil test is disabled.
If there is no stencil buffer,
no stencil modification can occur and it is as if
the stencil test always passes.
</para>
<para >
<citerefentry > <refentrytitle > glStencilFunc</refentrytitle> </citerefentry> is the same as
calling <citerefentry > <refentrytitle > glStencilFuncSeparate</refentrytitle> </citerefentry>
with <parameter > face</parameter> set to <constant > GL_FRONT_AND_BACK</constant> .
</para>
</refsect1>
<refsect1 id= "errors" > <title > Errors</title>
<para >
<constant > GL_INVALID_ENUM</constant> is generated if <parameter > func</parameter> is not one of the eight
accepted values.
</para>
</refsect1>
<refsect1 id= "associatedgets" > <title > Associated Gets</title>
<para >
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<citerefentry > <refentrytitle > glGet</refentrytitle> </citerefentry> with argument
<constant > GL_STENCIL_FUNC</constant> , <constant > GL_STENCIL_VALUE_MASK</constant> ,
<constant > GL_STENCIL_REF</constant> , <constant > GL_STENCIL_BACK_FUNC</constant> ,
<constant > GL_STENCIL_BACK_VALUE_MASK</constant> , <constant > GL_STENCIL_BACK_REF</constant> ,
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or <constant > GL_STENCIL_BITS</constant>
</para>
<para >
<citerefentry > <refentrytitle > glIsEnabled</refentrytitle> </citerefentry> with argument <constant > GL_STENCIL_TEST</constant>
</para>
</refsect1>
<refsect1 id= "seealso" > <title > See Also</title>
<para >
<citerefentry > <refentrytitle > glBlendFunc</refentrytitle> </citerefentry> ,
<citerefentry > <refentrytitle > glDepthFunc</refentrytitle> </citerefentry> ,
<citerefentry > <refentrytitle > glEnable</refentrytitle> </citerefentry> ,
<citerefentry > <refentrytitle > glLogicOp</refentrytitle> </citerefentry> ,
<citerefentry > <refentrytitle > glStencilFuncSeparate</refentrytitle> </citerefentry> ,
<citerefentry > <refentrytitle > glStencilMask</refentrytitle> </citerefentry> ,
<citerefentry > <refentrytitle > glStencilMaskSeparate</refentrytitle> </citerefentry> ,
<citerefentry > <refentrytitle > glStencilOp</refentrytitle> </citerefentry> ,
<citerefentry > <refentrytitle > glStencilOpSeparate</refentrytitle> </citerefentry>
</para>
</refsect1>
<refsect1 id= "Copyright" > <title > Copyright</title>
<para >
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Copyright <trademark class= "copyright" > </trademark> 1991-2006 Silicon Graphics, Inc.
Copyright <trademark class= "copyright" > </trademark> 2010-2013 Khronos Group.
This document is licensed under the SGI Free Software B License.
For details, see
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<ulink url= "http://oss.sgi.com/projects/FreeB/" > http://oss.sgi.com/projects/FreeB/</ulink> .
</para>
</refsect1>
</refentry>
