<!DOCTYPE refentry [ <!ENTITY % mathent SYSTEM "math.ent"> %mathent; ]>

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<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>&#x2211;</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>
    </refsect1>
    <refsect1 xml:id="versions"><title>Version Support</title>
        <informaltable>
            <tgroup cols="3" 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='10']/*)"/>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
    </refsect1>
    <refsect1 xml:id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>gl_FragCoord</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>
