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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
"http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
<refentry id="gluTessVertex">
<refmeta>
<refmetainfo>
<copyright>
<year>1991-2006</year>
<holder>Silicon Graphics, Inc.</holder>
</copyright>
</refmetainfo>
<refentrytitle>gluTessVertex</refentrytitle>
<manvolnum>3G</manvolnum>
</refmeta>
<refnamediv>
<refname>gluTessVertex</refname>
<refpurpose>specify a vertex on a polygon</refpurpose>
</refnamediv>
<refsynopsisdiv><title>C Specification</title>
<funcsynopsis>
<funcprototype>
<funcdef>void <function>gluTessVertex</function></funcdef>
<paramdef>GLUtesselator* <parameter>tess</parameter></paramdef>
<paramdef>GLdouble * <parameter>location</parameter></paramdef>
<paramdef>GLvoid* <parameter>data</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<!-- eqn: ignoring delim $$ -->
<refsect1 id="parameters"><title>Parameters</title>
<variablelist>
<varlistentry>
<term><parameter>tess</parameter></term>
<listitem>
<para>
Specifies the tessellation object (created with <citerefentry><refentrytitle>gluNewTess</refentrytitle></citerefentry>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>location</parameter></term>
<listitem>
<para>
Specifies the location of the vertex.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>data</parameter></term>
<listitem>
<para>
Specifies an opaque pointer passed back to the program with the vertex callback
(as specified by <citerefentry><refentrytitle>gluTessCallback</refentrytitle></citerefentry>).
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id="description"><title>Description</title>
<para>
<function>gluTessVertex</function> describes a vertex on a polygon that the program defines. Successive
<function>gluTessVertex</function> calls describe a closed contour. For example,
to describe a quadrilateral, <function>gluTessVertex</function> should be called four times.
<function>gluTessVertex</function> can only be called between <citerefentry><refentrytitle>gluTessBeginContour</refentrytitle></citerefentry> and
<citerefentry><refentrytitle>gluTessEndContour</refentrytitle></citerefentry>.
</para>
<para>
<parameter>data</parameter> normally points to a structure containing the vertex
location, as well as other per-vertex attributes such as color and normal.
This pointer is passed back to the user through the <constant>GLU_TESS_VERTEX</constant>
or <constant>GLU_TESS_VERTEX_DATA</constant> callback after tessellation
(see the <citerefentry><refentrytitle>gluTessCallback</refentrytitle></citerefentry> reference page).
</para>
</refsect1>
<refsect1 id="example"><title>Example</title>
<para>
A quadrilateral with a triangular hole in it can be described as follows:
<programlisting>
gluTessBeginPolygon(tobj, NULL);
gluTessBeginContour(tobj);
gluTessVertex(tobj, v1, v1);
gluTessVertex(tobj, v2, v2);
gluTessVertex(tobj, v3, v3);
gluTessVertex(tobj, v4, v4);
gluTessEndContour(tobj);
gluTessBeginContour(tobj);
gluTessVertex(tobj, v5, v5);
gluTessVertex(tobj, v6, v6);
gluTessVertex(tobj, v7, v7);
gluTessEndContour(tobj);
gluTessEndPolygon(tobj);
</programlisting>
</para>
</refsect1>
<refsect1 id="notes"><title>Notes</title>
<para>
It is a common error to use a local variable for <parameter>location</parameter> or <parameter>data</parameter> and store
values into it as part of a loop.
For example:
<programlisting>
for (i = 0; i < NVERTICES; ++i) {
GLdouble data[3];
data[0] = vertex[i][0];
data[1] = vertex[i][1];
data[2] = vertex[i][2];
gluTessVertex(tobj, data, data);
}
</programlisting>
</para>
<para>
This doesn't work.
Because the pointers specified by <parameter>location</parameter> and <parameter>data</parameter> might not be
dereferenced until <citerefentry><refentrytitle>gluTessEndPolygon</refentrytitle></citerefentry> is executed,
all the vertex coordinates but the very last set could be overwritten
before tessellation begins.
</para>
<para>
Two common symptoms of this problem are when the data consists of a single
point (when a local variable is used for <parameter>data</parameter>) and a
<constant>GLU_TESS_NEED_COMBINE_CALLBACK</constant> error (when a local variable is
used for <parameter>location</parameter>).
</para>
</refsect1>
<refsect1 id="seealso"><title>See Also</title>
<para>
<citerefentry><refentrytitle>gluNewTess</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>gluTessBeginContour</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>gluTessBeginPolygon</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>gluTessCallback</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>gluTessEndPolygon</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>gluTessNormal</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>gluTessProperty</refentrytitle></citerefentry>
</para>
</refsect1>
<refsect1 id="Copyright"><title>Copyright</title>
<para>
Copyright <trademark class="copyright"></trademark> 1991-2006
Silicon Graphics, Inc. This document is licensed under the SGI
Free Software B License. For details, see
<ulink url="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</ulink>.
</para>
</refsect1>
</refentry>