
<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 &lt; 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>
