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OpenGL ES 1.0 and EGL 1.0 reference manual and man pages (written by
Claude Knaus, based on OpenGL SI man pages).

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook V4.2//EN" "DTD/docbook/docbookx.dtd">

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<refentry id="glPolygonOffset">
  <refmeta>
    <refentrytitle>glPolygonOffset</refentrytitle>
    <manvolnum>3G</manvolnum>
  </refmeta>

  <refnamediv>
    <refdescriptor>glPolygonOffset</refdescriptor>
    <refname>glPolygonOffset</refname>
    <refname>glPolygonOffsetx</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>
      <funcprototype>
        <funcdef>void <function>glPolygonOffsetx</function></funcdef>
        <paramdef>GLfixed <parameter>factor</parameter></paramdef>
        <paramdef>GLfixed <parameter>units</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <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>
    <title>Description</title>

    <para>When <constant>GL_POLYGON_OFFSET_FILL</constant>
    is enabled, each fragment's <parameter>depth</parameter>
    value will be offset after it is interpolated from the 
    <parameter>depth</parameter>
    values of the appropriate vertices. The value of the offset is 
    <inlineequation><math>
      <mi>m</mi>
      <mo>*</mo>
      <mi>factor</mi>
      <mo>+</mo>
      <mi>r</mi>
      <mo>*</mo>
      <mi>units</mi>
    </math></inlineequation>,
    where <replaceable>m</replaceable>
    is a measurement of the change in depth relative to the screen
    area of the polygon, and <replaceable>r</replaceable>
    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 for applying decals to surfaces.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para>
    <citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>, 
    <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>
    </para>
  </refsect1>
</refentry>