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300 lines
14 KiB
XML
300 lines
14 KiB
XML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
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"http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
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<refentry id="glStencilFunc">
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<refmeta>
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<refmetainfo>
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<copyright>
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<year>1991-2006</year>
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<holder>Silicon Graphics, Inc.</holder>
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</copyright>
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</refmetainfo>
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<refentrytitle>glStencilFunc</refentrytitle>
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<manvolnum>3G</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>glStencilFunc</refname>
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<refpurpose>set front and back function and reference value for stencil testing</refpurpose>
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</refnamediv>
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<!-- eqn: ignoring delim $$ -->
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<refsynopsisdiv><title>C Specification</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>void <function>glStencilFunc</function></funcdef>
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<paramdef>GLenum <parameter>func</parameter></paramdef>
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<paramdef>GLint <parameter>ref</parameter></paramdef>
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<paramdef>GLuint <parameter>mask</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1 id="parameters"><title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><parameter>func</parameter></term>
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<listitem>
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<para>
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Specifies the test function.
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Eight symbolic constants are valid:
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<constant>GL_NEVER</constant>,
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<constant>GL_LESS</constant>,
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<constant>GL_LEQUAL</constant>,
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<constant>GL_GREATER</constant>,
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<constant>GL_GEQUAL</constant>,
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<constant>GL_EQUAL</constant>,
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<constant>GL_NOTEQUAL</constant>, and
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<constant>GL_ALWAYS</constant>. The initial value is <constant>GL_ALWAYS</constant>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>ref</parameter></term>
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<listitem>
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<para>
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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>
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<!-- eqn: [0, 2 sup n - 1]:-->
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<mml:mfenced open="[" close="]">
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<mml:mn>0</mml:mn>
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<mml:mrow>
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<mml:msup><mml:mn>2</mml:mn>
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<mml:mi mathvariant="italic">n</mml:mi>
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</mml:msup>
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<mml:mo>-</mml:mo>
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<mml:mn>1</mml:mn>
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</mml:mrow>
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</mml:mfenced>
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</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>
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is the number of bitplanes in the stencil buffer. The
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initial value is 0.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>mask</parameter></term>
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<listitem>
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<para>
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Specifies a mask that is ANDed with both the reference value
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and the stored stencil value when the test is done. The initial value
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is all 1's.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1 id="description"><title>Description</title>
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<para>
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Stenciling,
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like depth-buffering,
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enables and disables drawing on a per-pixel basis.
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Stencil planes are first drawn into using GL drawing primitives, then
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geometry and images are rendered using the stencil planes to mask out
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portions of the screen.
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Stenciling is typically used in multipass rendering algorithms
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to achieve special effects,
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such as decals,
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outlining,
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and constructive solid geometry rendering.
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</para>
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<para>
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The stencil test conditionally eliminates a pixel based on the outcome
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of a comparison between the reference value
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and the value in the stencil buffer.
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To enable and disable the test, call <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry> and <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry>
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with argument <constant>GL_STENCIL_TEST</constant>.
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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>.
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</para>
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<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
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and back stencil state to the same values. Use <citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry>
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to set front and back stencil state to different values.
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</para>
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<para>
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<parameter>func</parameter> is a symbolic constant that determines the stencil comparison function.
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It accepts one of eight values,
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shown in the following list.
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<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>
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<!-- eqn: [0, 2 sup n - 1]:-->
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<mml:mfenced open="[" close="]">
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<mml:mn>0</mml:mn>
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<mml:mrow>
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<mml:msup><mml:mn>2</mml:mn>
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<mml:mi mathvariant="italic">n</mml:mi>
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</mml:msup>
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<mml:mo>-</mml:mo>
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<mml:mn>1</mml:mn>
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</mml:mrow>
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</mml:mfenced>
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</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>
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is the number of bitplanes in the stencil buffer.
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<parameter>mask</parameter> is bitwise ANDed with both the reference value
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and the stored stencil value,
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with the ANDed values participating in the comparison.
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</para>
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<para>
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If <emphasis>stencil</emphasis> represents the value stored in the corresponding
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stencil buffer location,
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the following list shows the effect of each comparison function
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that can be specified by <parameter>func</parameter>.
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Only if the comparison succeeds is the pixel passed through
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to the next stage in the rasterization process
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(see <citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry>).
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All tests treat <emphasis>stencil</emphasis> values as unsigned integers in the range
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<inlineequation><mml:math>
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<!-- eqn: [0, 2 sup n - 1]:-->
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<mml:mfenced open="[" close="]">
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<mml:mn>0</mml:mn>
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<mml:mrow>
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<mml:msup><mml:mn>2</mml:mn>
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<mml:mi mathvariant="italic">n</mml:mi>
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</mml:msup>
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<mml:mo>-</mml:mo>
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<mml:mn>1</mml:mn>
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</mml:mrow>
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</mml:mfenced>
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</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>
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is the number of bitplanes in the stencil buffer.
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</para>
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<para>
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The following values are accepted by <parameter>func</parameter>:
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</para>
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<variablelist>
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<varlistentry>
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<term><constant>GL_NEVER</constant></term>
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<listitem>
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<para>
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Always fails.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_LESS</constant></term>
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<listitem>
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<para>
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Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) < ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_LEQUAL</constant></term>
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<listitem>
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<para>
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Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) <= ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_GREATER</constant></term>
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<listitem>
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<para>
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Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) > ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_GEQUAL</constant></term>
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<listitem>
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<para>
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Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) >= ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_EQUAL</constant></term>
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<listitem>
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<para>
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Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) = ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_NOTEQUAL</constant></term>
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<listitem>
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<para>
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Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) != ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_ALWAYS</constant></term>
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<listitem>
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<para>
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Always passes.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1 id="notes"><title>Notes</title>
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<para>
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Initially, the stencil test is disabled.
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If there is no stencil buffer,
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no stencil modification can occur and it is as if
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the stencil test always passes.
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</para>
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<para>
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<citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry> is the same as
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calling <citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry>
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with <parameter>face</parameter> set to <constant>GL_FRONT_AND_BACK</constant>.
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</para>
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</refsect1>
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<refsect1 id="errors"><title>Errors</title>
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<para>
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<constant>GL_INVALID_ENUM</constant> is generated if <parameter>func</parameter> is not one of the eight
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accepted values.
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</para>
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<para>
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<constant>GL_INVALID_OPERATION</constant> is generated if <function>glStencilFunc</function>
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is executed between the execution of <citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry>
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and the corresponding execution of <citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>.
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</para>
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</refsect1>
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<refsect1 id="associatedgets"><title>Associated Gets</title>
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<para>
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<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument
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<constant>GL_STENCIL_FUNC</constant>, <constant>GL_STENCIL_VALUE_MASK</constant>,
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<constant>GL_STENCIL_REF</constant>, <constant>GL_STENCIL_BACK_FUNC</constant>,
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<constant>GL_STENCIL_BACK_VALUE_MASK</constant>, <constant>GL_STENCIL_BACK_REF</constant>,
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or <constant>GL_STENCIL_BITS</constant>
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</para>
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<para>
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<citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument <constant>GL_STENCIL_TEST</constant>
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</para>
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</refsect1>
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<refsect1 id="seealso"><title>See Also</title>
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<para>
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<citerefentry><refentrytitle>glAlphaFunc</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glBlendFunc</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glLogicOp</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glStencilMask</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glStencilMaskSeparate</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glStencilOpSeparate</refentrytitle></citerefentry>
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</para>
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</refsect1>
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<refsect1 id="Copyright"><title>Copyright</title>
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<para>
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Copyright <trademark class="copyright"></trademark> 1991-2006
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Silicon Graphics, Inc. This document is licensed under the SGI
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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>.
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</para>
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</refsect1>
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</refentry>
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