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<?xml version="1.0" encoding="UTF-8"?>
<!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>, <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>
<!-- eqn: factor * DZ + r * units:-->
<mml:mrow>
<mml:mrow>
<mml:mi mathvariant="italic">factor</mml:mi>
<mml:mo>×</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>×</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="notes"><title>Notes</title>
<para>
<function>glPolygonOffset</function> is available only if the GL version is 1.1 or greater.
</para>
<para>
<function>glPolygonOffset</function> has no effect on depth coordinates placed in the
feedback buffer.
</para>
<para>
<function>glPolygonOffset</function> has no effect on selection.
</para>
</refsect1>
<refsect1 id="errors"><title>Errors</title>
<para>
<constant>GL_INVALID_OPERATION</constant> is generated if <function>glPolygonOffset</function> is executed
between the execution of <citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry> and the corresponding
execution of <citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>.
</para>
</refsect1>
<refsect1 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 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>