
<refentry id="gluTessProperty">
    <refmeta>
        <refmetainfo>
            <copyright>
                <year>1991-2006</year>
                <holder>Silicon Graphics, Inc.</holder>
            </copyright>
        </refmetainfo>
        <refentrytitle>gluTessProperty</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>gluTessProperty</refname>
        <refpurpose>set a tessellation object property</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void <function>gluTessProperty</function></funcdef>
                <paramdef>GLUtesselator* <parameter>tess</parameter></paramdef>
                <paramdef>GLenum <parameter>which</parameter></paramdef>
                <paramdef>GLdouble <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>which</parameter></term>
            <listitem>
                <para>
                    Specifies the property to be set. Valid values are
                    <constant>GLU_TESS_WINDING_RULE</constant>,
                    <constant>GLU_TESS_BOUNDARY_ONLY</constant>, and
                    <constant>GLU_TESS_TOLERANCE</constant>.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>data</parameter></term>
            <listitem>
                <para>
                    Specifies the value of the indicated property.
                </para>
            </listitem>
        </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 id="description"><title>Description</title>
        <para>
            <function>gluTessProperty</function> is used to control properties stored in a tessellation object. These
            properties affect the way that the polygons are interpreted and rendered. 
            The legal values for <parameter>which</parameter> are as follows:
        </para>
        <variablelist>
            <varlistentry>
                <term><constant>GLU_TESS_WINDING_RULE</constant> </term>
                <listitem>
                    <para>
                        Determines which parts of the polygon are on the ``interior''. 
                        <parameter>data</parameter> may be set to one of <constant>GLU_TESS_WINDING_ODD</constant>,
                        <constant>GLU_TESS_WINDING_NONZERO</constant>, <constant>GLU_TESS_WINDING_POSITIVE</constant>,
                        <constant>GLU_TESS_WINDING_NEGATIVE</constant>, or <constant>GLU_TESS_WINDING_ABS_GEQ_TWO</constant>.
                    </para>
                    <para>
                        To understand how the winding rule works, consider that the input 
                        contours partition the plane into regions. The winding rule determines which
                        of these regions are inside the polygon.
                    </para>
                    <para>
                        For a single contour C, the winding number of a point x is simply the signed
                        number of revolutions we make around x as we travel once around C
                        (where CCW is positive). When there are several contours, the individual
                        winding numbers are summed. This procedure associates a signed integer 
                        value with each point x in the plane. Note that the winding number is the
                        same for all points in a single region.
                    </para>
                    <para>
                        The winding rule classifies a region as ``inside'' if its winding number 
                        belongs to the chosen category (odd, nonzero, positive, negative, or
                        absolute value of at least two). The previous GLU tessellator (prior to
                        GLU 1.2) used the ``odd'' rule. The ``nonzero'' rule is another common way to
                        define the interior. The other three rules are useful for polygon CSG
                        operations.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GLU_TESS_BOUNDARY_ONLY</constant> </term>
                <listitem>
                    <para>
                        Is a boolean value (``value'' should be set
                        to GL_TRUE or GL_FALSE). When set to GL_TRUE, a set of closed contours
                        separating the polygon interior and exterior are returned instead of a 
                        tessellation. Exterior contours are oriented CCW with respect to the
                        normal; interior contours are oriented CW. The <constant>GLU_TESS_BEGIN</constant>
                        and <constant>GLU_TESS_BEGIN_DATA</constant> callbacks use the type GL_LINE_LOOP for
                        each contour.
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term><constant>GLU_TESS_TOLERANCE</constant></term>
                <listitem>
                    <para>
                        Specifies a tolerance for merging features to reduce the size of the output.
                        For example, two vertices that are very close to each other might be
                        replaced by a single vertex. The tolerance is multiplied by the largest
                        coordinate magnitude of any input vertex; this specifies the maximum
                        distance that any feature can move as the result of a single merge
                        operation. If a single feature takes part in several merge operations, the
                        total distance moved could be larger.
                    </para>
                    <para>
                        Feature merging is completely optional; the tolerance is only a hint.
                        The implementation is free to merge in some cases and not in others, or to
                        never merge features at all. The initial tolerance is 0.
                    </para>
                    <para>
                        The current implementation merges vertices only if they are exactly 
                        coincident, regardless of the current tolerance. A vertex is spliced into
                        an edge only if the implementation is unable to distinguish which side of
                        the edge the vertex lies on. Two edges are merged only when both endpoints
                        are identical.
                    </para>
                </listitem>
            </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>gluGetTessProperty</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>gluNewTess</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>
