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211 lines
9.1 KiB
XML
211 lines
9.1 KiB
XML
<?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="glAlphaFunc">
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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>glAlphaFunc</refentrytitle>
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<manvolnum>3G</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>glAlphaFunc</refname>
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<refpurpose>specify the alpha test function</refpurpose>
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</refnamediv>
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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>glAlphaFunc</function></funcdef>
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<paramdef>GLenum <parameter>func</parameter></paramdef>
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<paramdef>GLclampf <parameter>ref</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 alpha comparison function.
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Symbolic constants
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<constant>GL_NEVER</constant>,
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<constant>GL_LESS</constant>,
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<constant>GL_EQUAL</constant>,
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<constant>GL_LEQUAL</constant>,
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<constant>GL_GREATER</constant>,
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<constant>GL_NOTEQUAL</constant>,
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<constant>GL_GEQUAL</constant>, and
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<constant>GL_ALWAYS</constant> are accepted. 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 that incoming alpha values are compared to.
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This value is clamped to the range
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<inlineequation><mml:math>
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<!-- eqn: [0,1]:-->
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<mml:mfenced open="[" close="]">
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<mml:mn>0</mml:mn>
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<mml:mn>1</mml:mn>
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</mml:mfenced>
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</mml:math></inlineequation>,
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where 0 represents the lowest possible alpha value
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and 1 the highest possible value.
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The initial reference value is 0.
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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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The alpha test discards fragments depending on the outcome of a comparison
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between an incoming fragment's alpha value and a constant reference value.
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<function>glAlphaFunc</function> specifies the reference value and the comparison function.
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The comparison is performed only if alpha testing is enabled. By
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default, it is not enabled.
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(See
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<citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry> and <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry> of <constant>GL_ALPHA_TEST</constant>.)
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</para>
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<para>
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<parameter>func</parameter> and <parameter>ref</parameter> specify the conditions under which
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the pixel is drawn.
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The incoming alpha value is compared to <parameter>ref</parameter>
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using the function specified by <parameter>func</parameter>.
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If the value passes the comparison,
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the incoming fragment is drawn
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if it also passes subsequent stencil and depth buffer tests.
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If the value fails the comparison,
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no change is made to the frame buffer at that pixel location. The
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comparison functions are as follows:
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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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Never passes.
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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 the incoming alpha value is less than the reference value.
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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 the incoming alpha value is equal to the reference value.
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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 the incoming alpha value is less than or equal to the reference value.
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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 the incoming alpha value is greater than the reference value.
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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 the incoming alpha value is not equal to the reference value.
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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 the incoming alpha value is greater than or equal to
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the reference value.
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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 (initial value).
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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<function>glAlphaFunc</function> operates on all pixel write operations,
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including those resulting from the scan conversion of points,
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lines,
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polygons,
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and bitmaps,
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and from pixel draw and copy operations.
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<function>glAlphaFunc</function> does not affect screen clear operations.
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</para>
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</refsect1>
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<refsect1 id="notes"><title>Notes</title>
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<para>
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Alpha testing is performed only in RGBA mode.
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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 an accepted value.
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</para>
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<para>
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<constant>GL_INVALID_OPERATION</constant> is generated if <function>glAlphaFunc</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 <constant>GL_ALPHA_TEST_FUNC</constant>
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</para>
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<para>
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<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_ALPHA_TEST_REF</constant>
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</para>
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<para>
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<citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument <constant>GL_ALPHA_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>glBlendFunc</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glClear</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>glStencilFunc</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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