120 lines
5.4 KiB
XML
120 lines
5.4 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
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"http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
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<refentry id="glPolygonOffset">
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<refmeta>
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<refmetainfo>
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<copyright>
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<year>1991-2006</year>
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<holder>Silicon Graphics, Inc.</holder>
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</copyright>
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</refmetainfo>
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<refentrytitle>glPolygonOffset</refentrytitle>
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<manvolnum>3G</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>glPolygonOffset</refname>
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<refpurpose>set the scale and units used to calculate depth values</refpurpose>
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</refnamediv>
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<refsynopsisdiv><title>C Specification</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>void <function>glPolygonOffset</function></funcdef>
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<paramdef>GLfloat <parameter>factor</parameter></paramdef>
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<paramdef>GLfloat <parameter>units</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<!-- eqn: ignoring delim $$ -->
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<refsect1 id="parameters"><title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><parameter>factor</parameter></term>
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<listitem>
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<para>
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Specifies a scale factor that is used to create a variable
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depth offset for each polygon. The initial value is 0.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>units</parameter></term>
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<listitem>
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<para>
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Is multiplied by an implementation-specific value to
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create a constant depth offset. The initial value is 0.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1 id="description"><title>Description</title>
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<para>
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When <constant>GL_POLYGON_OFFSET_FILL</constant>, <constant>GL_POLYGON_OFFSET_LINE</constant>, or
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<constant>GL_POLYGON_OFFSET_POINT</constant> is enabled, each
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fragment's <emphasis>depth</emphasis> value will be offset after it is interpolated
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from the <emphasis>depth</emphasis> values of the appropriate vertices.
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The value of the offset is
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<inlineequation><mml:math>
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<!-- eqn: factor * DZ + r * units: -->
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<mml:mrow>
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<mml:mrow>
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<mml:mi mathvariant="italic">factor</mml:mi>
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<mml:mo>×</mml:mo>
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<mml:mi mathvariant="italic">DZ</mml:mi>
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</mml:mrow>
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<mml:mo>+</mml:mo>
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<mml:mrow>
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<mml:mi mathvariant="italic">r</mml:mi>
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<mml:mo>×</mml:mo>
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<mml:mi mathvariant="italic">units</mml:mi>
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</mml:mrow>
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</mml:mrow>
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</mml:math></inlineequation>,
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where
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<inlineequation><mml:math>
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<!-- eqn: DZ: -->
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<mml:mi mathvariant="italic">DZ</mml:mi>
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</mml:math></inlineequation>
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is a measurement of the change in depth relative to the screen
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area of the polygon, and
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<inlineequation><mml:math><mml:mi mathvariant="italic">r</mml:mi></mml:math></inlineequation>
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is the smallest value that is guaranteed to
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produce a resolvable offset for a given implementation.
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The offset is added before the depth test is performed and before
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the value is written into the depth buffer.
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</para>
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<para>
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<function>glPolygonOffset</function> is useful for rendering hidden-line images, for applying decals
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to surfaces, and for rendering solids with highlighted edges.
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</para>
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</refsect1>
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<refsect1 id="associatedgets"><title>Associated Gets</title>
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<para>
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<citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument
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<constant>GL_POLYGON_OFFSET_FILL</constant>,
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<constant>GL_POLYGON_OFFSET_LINE</constant>,
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or <constant>GL_POLYGON_OFFSET_POINT</constant>.
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</para>
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<para>
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<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_POLYGON_OFFSET_FACTOR</constant> or
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<constant>GL_POLYGON_OFFSET_UNITS</constant>.
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</para>
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</refsect1>
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<refsect1 id="seealso"><title>See Also</title>
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<para>
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<citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry>
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</para>
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</refsect1>
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<refsect1 id="Copyright"><title>Copyright</title>
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<para>
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Copyright <trademark class="copyright"></trademark> 1991-2006
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Silicon Graphics, Inc. This document is licensed under the SGI
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Free Software B License. For details, see
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<ulink url="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</ulink>.
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</para>
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</refsect1>
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</refentry>
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