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<!DOCTYPE refentry [ <!ENTITY % mathent SYSTEM "math.ent"> %mathent; ]>
<!-- Converted by db4-upgrade version 1.1 -->
<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="gl_FrontFacing">
<info>
<copyright>
<year>2011-2014</year>
<holder>Khronos Group</holder>
</copyright>
</info>
<refmeta>
<refentrytitle>gl_FrontFacing</refentrytitle>
<manvolnum>3G</manvolnum>
</refmeta>
<refnamediv>
<refname>gl_FrontFacing</refname>
<refpurpose>indicates whether a primitive is front or back facing</refpurpose>
</refnamediv>
<refsynopsisdiv>
<title>Declaration</title>
<fieldsynopsis>
<modifier>in</modifier>
<type>bool</type>
<varname>gl_FrontFacing</varname>
</fieldsynopsis>
</refsynopsisdiv>
<refsect1 xml:id="description"><title>Description</title>
<para>
Available only in the fragment language, <varname>gl_FrontFacing</varname> is an input variable
whose value is <function>true</function> if the fragment belongs to a front-facing primitive and
false otherwise. The determination of whether a triangle primitive is front-facing is made by
examining the sign of the area of the triangle, including a possible reversal of this sign
as controlled by <varname>glFrontFace</varname>. One way to compute this area is:
</para>
<para>
<inlineequation><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
<mml:mi>a</mml:mi><mml:mo>=</mml:mo><mml:mfrac><mml:mn>1</mml:mn><mml:mn>2</mml:mn></mml:mfrac>
<mml:msubsup>
<mml:mo>∑</mml:mo>
<mml:mrow>
<mml:mi>j</mml:mi><mml:mo>=</mml:mo><mml:mn>0</mml:mn>
</mml:mrow>
<mml:mrow>
<mml:mi>n</mml:mi><mml:mo>-</mml:mo><mml:mn>1</mml:mn>
</mml:mrow>
</mml:msubsup>
<mml:msubsup>
<mml:mi>x</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi></mml:mrow>
</mml:msubsup>
<mml:msubsup>
<mml:mi>y</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mrow>
</mml:msubsup>
<mml:mo>-</mml:mo>
<mml:msubsup>
<mml:mi>x</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mrow>
</mml:msubsup>
<mml:msubsup>
<mml:mi>y</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi></mml:mrow>
</mml:msubsup>
</mml:math></inlineequation>
</para>
<para>
where
<inlineequation><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
<mml:msubsup>
<mml:mi>x</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi></mml:mrow>
</mml:msubsup>
</mml:math></inlineequation> and
<inlineequation><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
<mml:msubsup>
<mml:mi>y</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi></mml:mrow>
</mml:msubsup>
</mml:math></inlineequation> are
the x and y window coordinates of the <emphasis>i</emphasis>th vertex of the n-vertex polygon.
</para>
<para>
The sign of this computation is negated when the value of
<constant>GL_CLIP_ORIGIN</constant> (the clip volume origin, set
with
<citerefentry><refentrytitle>glClipControl</refentrytitle></citerefentry>)
is <constant>GL_UPPER_LEFT</constant>.
</para>
</refsect1>
<refsect1 xml:id="versions"><title>Version Support</title>
<informaltable>
<tgroup cols="13" align="left">
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="varhead.xml" xpointer="xpointer(/*/*)"/>
<tbody>
<row>
<entry>gl_FrontFacing</entry>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="version.xml" xpointer="xpointer(/*/*[@role='11']/*)"/>
</row>
</tbody>
</tgroup>
</informaltable>
</refsect1>
<refsect1 xml:id="seealso"><title>See Also</title>
<para>
<citerefentry><refentrytitle>gl_FragCoord</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glClipControl</refentrytitle></citerefentry>
</para>
</refsect1>
<refsect1 xml:id="Copyright"><title>Copyright</title>
<para>
Copyright <trademark class="copyright"/> 2011-2014 Khronos Group.
This material may be distributed subject to the terms and conditions set forth in
the Open Publication License, v 1.0, 8 June 1999.
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://opencontent.org/openpub/">http://opencontent.org/openpub/</link>.
</para>
</refsect1>
</refentry>
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