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<!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="glPolygonOffset">
    <refmeta>
        <refmetainfo>
            <copyright>
                <year>1991-2006</year>
                <holder>Silicon Graphics, Inc.</holder>
            </copyright>
        </refmetainfo>
        <refentrytitle>glPolygonOffset</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>glPolygonOffset</refname>
        <refpurpose>set the scale and units used to calculate depth values</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void <function>glPolygonOffset</function></funcdef>
                <paramdef>GLfloat <parameter>factor</parameter></paramdef>
                <paramdef>GLfloat <parameter>units</parameter></paramdef>
            </funcprototype>
        </funcsynopsis>
    </refsynopsisdiv>
    <!-- eqn: ignoring delim $$ -->
    <refsect1 id="parameters"><title>Parameters</title>
        <variablelist>
        <varlistentry>
            <term><parameter>factor</parameter></term>
            <listitem>
                <para>
                    Specifies a scale factor that is used to create a variable
                    depth offset for each polygon. The initial value is 0.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>units</parameter></term>
            <listitem>
                <para>
                    Is multiplied by an implementation-specific value to
                    create a constant depth offset. The initial value is 0.
                </para>
            </listitem>
        </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 id="description"><title>Description</title>
        <para>
            When <constant>GL_POLYGON_OFFSET_FILL</constant> is enabled, each
            fragment's <emphasis>depth</emphasis> value will be offset after it is interpolated
            from the <emphasis>depth</emphasis> values of the appropriate vertices.
            The value of the offset is 
            <inlineequation><mml:math>
                <!-- eqn: factor  *  DZ   +   r  *  units:-->
                <mml:mrow>
                    <mml:mrow>
                        <mml:mi mathvariant="italic">factor</mml:mi>
                        <mml:mo>&times;</mml:mo>
                        <mml:mi mathvariant="italic">DZ</mml:mi>
                    </mml:mrow>
                    <mml:mo>+</mml:mo>
                    <mml:mrow>
                        <mml:mi mathvariant="italic">r</mml:mi>
                        <mml:mo>&times;</mml:mo>
                        <mml:mi mathvariant="italic">units</mml:mi>
                    </mml:mrow>
                </mml:mrow>
            </mml:math></inlineequation>,
            where 
            <inlineequation><mml:math>
                <!-- eqn: DZ:-->
                <mml:mi mathvariant="italic">DZ</mml:mi>
            </mml:math></inlineequation>
            is a measurement of the change in depth relative to the screen
            area of the polygon, and 
            <inlineequation><mml:math><mml:mi mathvariant="italic">r</mml:mi></mml:math></inlineequation>
            is the smallest value that is guaranteed to
            produce a resolvable offset for a given implementation.
            The offset is added before the depth test is performed and before
            the value is written into the depth buffer.
        </para>
        <para>
            <function>glPolygonOffset</function> is useful for rendering hidden-line images, for applying decals
            to surfaces, and for rendering solids with highlighted edges.
        </para>
    </refsect1>
    <refsect1 id="associatedgets"><title>Associated Gets</title>
        <para>
            <citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument
            <constant>GL_POLYGON_OFFSET_FILL</constant>.
        </para>
        <para>
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_POLYGON_OFFSET_FACTOR</constant> or
            <constant>GL_POLYGON_OFFSET_UNITS</constant>.
        </para>
    </refsect1>
    <refsect1 id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glIsEnabled</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>
