<!DOCTYPE refentry [ <!ENTITY % mathent SYSTEM "math.ent"> %mathent; ]>

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<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="glPolygonOffset">
    <info>
        <copyright>
            <year>1991-2006</year>
            <holder>Silicon Graphics, Inc.</holder>
        </copyright>
        <copyright>
            <year>2010-2013</year>
            <holder>Khronos Group</holder>
        </copyright>
    </info>
    <refmeta>
        <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>
    <refsect1 xml: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 xml:id="description"><title>Description</title>
        <para>
            When <constant>GL_POLYGON_OFFSET_FILL</constant>, <constant>GL_POLYGON_OFFSET_LINE</constant>, or
            <constant>GL_POLYGON_OFFSET_POINT</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 xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
                <!-- 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 xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
                <!-- 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 xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><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 xml:id="associatedgets"><title>Associated Gets</title>
        <para>
            <citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument
            <constant>GL_POLYGON_OFFSET_FILL</constant>,
            <constant>GL_POLYGON_OFFSET_LINE</constant>,
            or <constant>GL_POLYGON_OFFSET_POINT</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 xml: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 xml:id="Copyright"><title>Copyright</title>
        <para>
            Copyright <trademark class="copyright"/> 1991-2006 Silicon Graphics, Inc.
            Copyright <trademark class="copyright"/> 2010-2013 Khronos Group.
            This document is licensed under the SGI Free Software B License.
            For details, see
            <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</link>.
        </para>
    </refsect1>
</refentry>
