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144 lines
6.5 KiB
XML
144 lines
6.5 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="gluTessVertex">
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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>gluTessVertex</refentrytitle>
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<manvolnum>3G</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>gluTessVertex</refname>
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<refpurpose>specify a vertex on a polygon</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>gluTessVertex</function></funcdef>
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<paramdef>GLUtesselator* <parameter>tess</parameter></paramdef>
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<paramdef>GLdouble * <parameter>location</parameter></paramdef>
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<paramdef>GLvoid* <parameter>data</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>location</parameter></term>
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<listitem>
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<para>
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Specifies the location of the vertex.
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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>data</parameter></term>
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<listitem>
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<para>
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Specifies an opaque pointer passed back to the program with the vertex callback
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(as specified by <citerefentry><refentrytitle>gluTessCallback</refentrytitle></citerefentry>).
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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>gluTessVertex</function> describes a vertex on a polygon that the program defines. Successive
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<function>gluTessVertex</function> calls describe a closed contour. For example,
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to describe a quadrilateral, <function>gluTessVertex</function> should be called four times.
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<function>gluTessVertex</function> can only be called between <citerefentry><refentrytitle>gluTessBeginContour</refentrytitle></citerefentry> and
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<citerefentry><refentrytitle>gluTessEndContour</refentrytitle></citerefentry>.
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</para>
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<para>
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<parameter>data</parameter> normally points to a structure containing the vertex
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location, as well as other per-vertex attributes such as color and normal.
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This pointer is passed back to the user through the <constant>GLU_TESS_VERTEX</constant>
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or <constant>GLU_TESS_VERTEX_DATA</constant> callback after tessellation
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(see the <citerefentry><refentrytitle>gluTessCallback</refentrytitle></citerefentry> reference page).
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</para>
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</refsect1>
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<refsect1 id="example"><title>Example</title>
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<para>
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A quadrilateral with a triangular hole in it can be described as follows:
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<programlisting>
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gluTessBeginPolygon(tobj, NULL);
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gluTessBeginContour(tobj);
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gluTessVertex(tobj, v1, v1);
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gluTessVertex(tobj, v2, v2);
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gluTessVertex(tobj, v3, v3);
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gluTessVertex(tobj, v4, v4);
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gluTessEndContour(tobj);
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gluTessBeginContour(tobj);
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gluTessVertex(tobj, v5, v5);
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gluTessVertex(tobj, v6, v6);
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gluTessVertex(tobj, v7, v7);
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gluTessEndContour(tobj);
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gluTessEndPolygon(tobj);
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</programlisting>
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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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It is a common error to use a local variable for <parameter>location</parameter> or <parameter>data</parameter> and store
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values into it as part of a loop.
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For example:
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<programlisting>
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for (i = 0; i < NVERTICES; ++i) {
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GLdouble data[3];
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data[0] = vertex[i][0];
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data[1] = vertex[i][1];
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data[2] = vertex[i][2];
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gluTessVertex(tobj, data, data);
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}
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</programlisting>
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</para>
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<para>
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This doesn't work.
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Because the pointers specified by <parameter>location</parameter> and <parameter>data</parameter> might not be
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dereferenced until <citerefentry><refentrytitle>gluTessEndPolygon</refentrytitle></citerefentry> is executed,
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all the vertex coordinates but the very last set could be overwritten
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before tessellation begins.
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</para>
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<para>
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Two common symptoms of this problem are when the data consists of a single
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point (when a local variable is used for <parameter>data</parameter>) and a
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<constant>GLU_TESS_NEED_COMBINE_CALLBACK</constant> error (when a local variable is
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used for <parameter>location</parameter>).
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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>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>gluTessCallback</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluTessEndPolygon</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluTessNormal</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>gluTessProperty</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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