102 lines
4.8 KiB
XML
102 lines
4.8 KiB
XML
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<?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="glFrontFace">
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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>glFrontFace</refentrytitle>
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<manvolnum>3G</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>glFrontFace</refname>
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<refpurpose>define front- and back-facing polygons</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>glFrontFace</function></funcdef>
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<paramdef>GLenum <parameter>mode</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>mode</parameter></term>
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<listitem>
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<para>
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Specifies the orientation of front-facing polygons.
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<constant>GL_CW</constant> and <constant>GL_CCW</constant> are accepted.
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The initial value is <constant>GL_CCW</constant>.
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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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In a scene composed entirely of opaque closed surfaces,
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back-facing polygons are never visible.
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Eliminating these invisible polygons has the obvious benefit
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of speeding up the rendering of the image.
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To enable and disable elimination of back-facing polygons, call <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>
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and <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry> with argument <constant>GL_CULL_FACE</constant>.
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</para>
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<para>
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The projection of a polygon to window coordinates is said to have
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clockwise winding if an imaginary object following the path
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from its first vertex,
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its second vertex,
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and so on,
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to its last vertex,
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and finally back to its first vertex,
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moves in a clockwise direction about the interior of the polygon.
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The polygon's winding is said to be counterclockwise if the imaginary
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object following the same path moves in a counterclockwise direction
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about the interior of the polygon.
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<function>glFrontFace</function> specifies whether polygons with clockwise winding in window coordinates,
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or counterclockwise winding in window coordinates,
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are taken to be front-facing.
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Passing <constant>GL_CCW</constant> to <parameter>mode</parameter> selects counterclockwise polygons as
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front-facing;
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<constant>GL_CW</constant> selects clockwise polygons as front-facing.
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By default, counterclockwise polygons are taken to be front-facing.
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</para>
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</refsect1>
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<refsect1 id="errors"><title>Errors</title>
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<para>
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<constant>GL_INVALID_ENUM</constant> is generated if <parameter>mode</parameter> is not an accepted value.
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</para>
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<para>
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<constant>GL_INVALID_OPERATION</constant> is generated if <function>glFrontFace</function>
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is executed between the execution of <citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry>
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and the corresponding execution of <citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>.
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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>glGet</refentrytitle></citerefentry> with argument <constant>GL_FRONT_FACE</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>glCullFace</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glLightModel</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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