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380 lines
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380 lines
21 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="gluTessCallback">
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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>gluTessCallback</refentrytitle>
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<manvolnum>3G</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>gluTessCallback</refname>
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<refpurpose>define a callback for a tessellation object</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>gluTessCallback</function></funcdef>
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<paramdef>GLUtesselator* <parameter>tess</parameter></paramdef>
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<paramdef>GLenum <parameter>which</parameter></paramdef>
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<paramdef>_GLUfuncptr <parameter>CallBackFunc</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<!-- eqn: ignoring delim $$ -->
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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>tess</parameter></term>
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<listitem>
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<para>
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Specifies the tessellation object (created with <citerefentry><refentrytitle>gluNewTess</refentrytitle></citerefentry>).
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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>which</parameter></term>
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<listitem>
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<para>
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Specifies the callback being defined. The following values are valid:
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<constant>GLU_TESS_BEGIN</constant>,
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<constant>GLU_TESS_BEGIN_DATA</constant>,
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<constant>GLU_TESS_EDGE_FLAG</constant>,
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<constant>GLU_TESS_EDGE_FLAG_DATA</constant>,
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<constant>GLU_TESS_VERTEX</constant>,
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<constant>GLU_TESS_VERTEX_DATA</constant>,
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<constant>GLU_TESS_END</constant>,
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<constant>GLU_TESS_END_DATA</constant>,
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<constant>GLU_TESS_COMBINE</constant>,
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<constant>GLU_TESS_COMBINE_DATA</constant>,
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<constant>GLU_TESS_ERROR</constant>, and
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<constant>GLU_TESS_ERROR_DATA</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>CallBackFunc</parameter></term>
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<listitem>
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<para>
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Specifies the function to be called.
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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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<function>gluTessCallback</function> is used to indicate a callback to be used by a tessellation object.
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If the specified callback is already defined, then it is replaced. If
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<parameter>CallBackFunc</parameter> is NULL, then the existing callback becomes undefined.
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</para>
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<para>
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These callbacks are used by the tessellation object to describe how a
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polygon specified by the user is broken into triangles. Note that there
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are two versions of each callback: one with user-specified polygon data
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and one without. If both versions of a particular callback are specified,
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then the callback with user-specified polygon data will be used. Note
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that the <emphasis>polygon_data</emphasis> parameter used by some of the functions is
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a copy of the pointer that was specified when
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<citerefentry><refentrytitle>gluTessBeginPolygon</refentrytitle></citerefentry> was called. The legal callbacks are as follows:
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</para>
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<variablelist>
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<varlistentry>
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<term><constant>GLU_TESS_BEGIN</constant></term>
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<listitem>
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<para>
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The begin callback is invoked like <citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry> to indicate the start of
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a (triangle) primitive. The function takes a single argument of type
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GLenum. If the <constant>GLU_TESS_BOUNDARY_ONLY</constant> property is set to
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<constant>GLU_FALSE</constant>, then the argument is set to either
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<constant>GLU_TRIANGLE_FAN</constant>, <constant>GLU_TRIANGLE_STRIP</constant>, or <constant>GLU_TRIANGLES</constant>.
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If the <constant>GLU_TESS_BOUNDARY_ONLY</constant> property is set to <constant>GLU_TRUE</constant>,
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then the argument will be set to <constant>GLU_LINE_LOOP</constant>. The function
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prototype for this callback is:
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<programlisting>
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void begin( GLenum type );
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</programlisting>
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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>GLU_TESS_BEGIN_DATA</constant></term>
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<listitem>
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<para>
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The same as the <constant>GLU_TESS_BEGIN</constant> callback except that it
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takes an additional pointer argument. This pointer is identical to the
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opaque pointer provided when
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<citerefentry><refentrytitle>gluTessBeginPolygon</refentrytitle></citerefentry> was called. The function prototype for this callback
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is:
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<programlisting>
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void beginData( GLenum type, void *polygon_data );
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</programlisting>
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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>GLU_TESS_EDGE_FLAG</constant></term>
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<listitem>
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<para>
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The edge flag callback is similar to <citerefentry><refentrytitle>glEdgeFlag</refentrytitle></citerefentry>. The function
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takes a single boolean flag that indicates which edges lie on the
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polygon boundary. If the flag is <constant>GLU_TRUE</constant>, then each vertex
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that follows begins an edge that lies on the polygon boundary, that is,
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an edge that separates an interior region from an exterior one.
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If the flag is <constant>GLU_FALSE</constant>, then each vertex that follows begins an edge
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that lies in the polygon interior. The edge flag callback (if defined) is
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invoked before the first vertex callback.
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</para>
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<para>
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Since triangle fans and triangle strips do not support edge flags, the begin
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callback is not called with <constant>GLU_TRIANGLE_FAN</constant> or <constant>GLU_TRIANGLE_STRIP</constant>
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if a non-NULL edge flag callback is provided. (If the callback is
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initialized to NULL, there is no impact on performance). Instead, the fans and
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strips are converted to independent triangles. The function prototype
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for this callback is:
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<programlisting>
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void edgeFlag( GLboolean flag );
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</programlisting>
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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>GLU_TESS_EDGE_FLAG_DATA</constant></term>
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<listitem>
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<para>
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The same as the <constant>GLU_TESS_EDGE_FLAG</constant> callback except that it takes an additional pointer
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argument. This pointer is identical to the opaque pointer provided when
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<citerefentry><refentrytitle>gluTessBeginPolygon</refentrytitle></citerefentry> was called. The function prototype for this callback
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is:
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<programlisting>
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void edgeFlagData( GLboolean flag, void *polygon_data );
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</programlisting>
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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>GLU_TESS_VERTEX</constant></term>
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<listitem>
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<para>
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The vertex callback is invoked between the begin and end callbacks.
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It is similar to <citerefentry><refentrytitle>glVertex</refentrytitle></citerefentry>, and it defines the vertices of the triangles
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created by the tessellation process. The function
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takes a pointer as its only argument. This pointer is identical to
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the opaque pointer provided by the user when the vertex was described
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(see <citerefentry><refentrytitle>gluTessVertex</refentrytitle></citerefentry>). The function prototype for this callback is:
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<programlisting>
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void vertex( void *vertex_data );
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</programlisting>
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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>GLU_TESS_VERTEX_DATA</constant></term>
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<listitem>
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<para>
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The same as the <constant>GLU_TESS_VERTEX</constant> callback except that it takes an additional pointer
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argument. This pointer is identical to the opaque pointer provided when
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<citerefentry><refentrytitle>gluTessBeginPolygon</refentrytitle></citerefentry> was called. The function prototype for this callback
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is:
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<programlisting>
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void vertexData( void *vertex_data, void *polygon_data );
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</programlisting>
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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>GLU_TESS_END</constant></term>
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<listitem>
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<para>
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The end callback serves the same purpose as <citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>. It indicates the
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end of a primitive and it takes no arguments. The function prototype for this
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callback is:
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<programlisting>
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void end( void );
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</programlisting>
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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>GLU_TESS_END_DATA</constant></term>
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<listitem>
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<para>
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The same as the <constant>GLU_TESS_END</constant> callback except that it takes an additional pointer
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argument. This pointer is identical to the opaque pointer provided when
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<citerefentry><refentrytitle>gluTessBeginPolygon</refentrytitle></citerefentry> was called. The function prototype for this callback
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is:
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<programlisting>
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void endData( void *polygon_data );
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</programlisting>
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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>GLU_TESS_COMBINE</constant></term>
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<listitem>
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<para>
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The combine callback is called to create a new vertex when the tessellation
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detects an intersection or wishes to merge features. The function takes
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four arguments: an array of three elements each of type GLdouble, an array
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of four pointers, an array of four elements each of type GLfloat, and a
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pointer to a pointer. The prototype is:
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<programlisting>
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void combine( GLdouble coords[3], void *vertex_data[4],
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GLfloat weight[4], void **outData );
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</programlisting>
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</para>
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<para>
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The vertex is defined as a linear combination of up to four existing vertices,
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stored in <emphasis>vertex_data</emphasis>. The coefficients of the linear
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combination are given by <emphasis>weight</emphasis>; these weights always add up to 1.
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All vertex pointers are valid even when some of the weights are 0.
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<emphasis>coords</emphasis> gives the location of the new vertex.
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</para>
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<para>
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The user must allocate another vertex, interpolate parameters using
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<emphasis>vertex_data</emphasis> and <emphasis>weight</emphasis>, and return the new vertex pointer in
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<emphasis>outData</emphasis>. This handle is supplied during rendering callbacks.
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The user is responsible for freeing the memory some time after
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<citerefentry><refentrytitle>gluTessEndPolygon</refentrytitle></citerefentry> is called.
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</para>
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<para>
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For example, if the polygon lies in an arbitrary plane in 3-space,
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and a color is associated with each vertex, the
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<constant>GLU_TESS_COMBINE</constant> callback might look like this:
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<programlisting>
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void myCombine( GLdouble coords[3], VERTEX *d[4],
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GLfloat w[4], VERTEX **dataOut )
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{
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VERTEX *new = new_vertex();
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new->x = coords[0];
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new->y = coords[1];
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new->z = coords[2];
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new->r = w[0]*d[0]->r + w[1]*d[1]->r + w[2]*d[2]->r + w[3]*d[3]->r;
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new->g = w[0]*d[0]->g + w[1]*d[1]->g + w[2]*d[2]->g + w[3]*d[3]->g;
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new->b = w[0]*d[0]->b + w[1]*d[1]->b + w[2]*d[2]->b + w[3]*d[3]->b;
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new->a = w[0]*d[0]->a + w[1]*d[1]->a + w[2]*d[2]->a + w[3]*d[3]->a;
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*dataOut = new;
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}
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</programlisting>
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</para>
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<para>
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If the tessellation detects an intersection, then the <constant>GLU_TESS_COMBINE</constant> or
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<constant>GLU_TESS_COMBINE_DATA</constant> callback (see below) must be defined, and it must
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write a non-NULL pointer into <emphasis>dataOut</emphasis>. Otherwise the
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<constant>GLU_TESS_NEED_COMBINE_CALLBACK</constant> error occurs, and no
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output is generated.
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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>GLU_TESS_COMBINE_DATA</constant></term>
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<listitem>
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<para>
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The same as the <constant>GLU_TESS_COMBINE</constant> callback except that it takes an additional pointer
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argument. This pointer is identical to the opaque pointer provided when
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<citerefentry><refentrytitle>gluTessBeginPolygon</refentrytitle></citerefentry> was called. The function prototype for this callback
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is:
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<programlisting>
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void combineData( GLdouble coords[3], void *vertex_data[4],
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GLfloat weight[4], void **outData,
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void *polygon_data );
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</programlisting>
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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>GLU_TESS_ERROR</constant></term>
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<listitem>
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<para>
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The error callback is called when an error is encountered. The one argument
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is of type GLenum; it indicates the specific error that occurred and will be
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set to one of <constant>GLU_TESS_MISSING_BEGIN_POLYGON</constant>, <constant>GLU_TESS_MISSING_END_POLYGON</constant>,
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<constant>GLU_TESS_MISSING_BEGIN_CONTOUR</constant>, <constant>GLU_TESS_MISSING_END_CONTOUR</constant>,
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<constant>GLU_TESS_COORD_TOO_LARGE</constant>, <constant>GLU_TESS_NEED_COMBINE_CALLBACK</constant>, or
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<constant>GLU_OUT_OF_MEMORY</constant>. Character
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strings describing these errors can be retrieved with the
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<citerefentry><refentrytitle>gluErrorString</refentrytitle></citerefentry> call. The function prototype for this callback is:
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<programlisting>
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void error( GLenum errno );
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</programlisting>
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</para>
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<para>
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The GLU library will recover from the first four
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errors by inserting the missing call(s).
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<constant>GLU_TESS_COORD_TOO_LARGE</constant> indicates that some vertex coordinate exceeded
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the predefined constant <constant>GLU_TESS_MAX_COORD</constant> in absolute value, and
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that the value has been clamped. (Coordinate values must be small
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enough so that two can be multiplied together without overflow.)
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<constant>GLU_TESS_NEED_COMBINE_CALLBACK</constant> indicates that the tessellation
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detected an intersection between two edges in the input data, and the
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<constant>GLU_TESS_COMBINE</constant> or <constant>GLU_TESS_COMBINE_DATA</constant> callback was
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not provided. No output is generated. <constant>GLU_OUT_OF_MEMORY</constant> indicates
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that there is not enough memory so no output is generated.
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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>GLU_TESS_ERROR_DATA</constant></term>
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<listitem>
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<para>
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The same as the <constant>GLU_TESS_ERROR</constant> callback except that it takes an additional pointer
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argument. This pointer is identical to the opaque pointer provided when
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<citerefentry><refentrytitle>gluTessBeginPolygon</refentrytitle></citerefentry> was called. The function prototype for this callback
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is:
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<programlisting>
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void errorData( GLenum errno, void *polygon_data );
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</programlisting>
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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="example"><title>Example</title>
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<para>
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Polygons tessellated can be rendered directly like this:
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<programlisting>
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gluTessCallback(tobj, GLU_TESS_BEGIN, glBegin);
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gluTessCallback(tobj, GLU_TESS_VERTEX, glVertex3dv);
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gluTessCallback(tobj, GLU_TESS_END, glEnd);
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gluTessCallback(tobj, GLU_TESS_COMBINE, myCombine);
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gluTessBeginPolygon(tobj, NULL);
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gluTessBeginContour(tobj);
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gluTessVertex(tobj, v, v);
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...
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gluTessEndContour(tobj);
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gluTessEndPolygon(tobj);
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</programlisting>
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Typically, the tessellated polygon should be stored in a display list so that
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it does not need to be retessellated every time it is rendered.
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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>gluErrorString</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluNewTess</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluTessBeginContour</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluTessBeginPolygon</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluTessNormal</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluTessProperty</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluTessVertex</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glEdgeFlag</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glVertex</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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