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<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="gl_Position">
    <info>
        <copyright>
            <year>2011-2013</year>
            <holder>Khronos Group</holder>
        </copyright>
    </info>
    <refmeta>
        <refentrytitle>gl_Position</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>gl_Position</refname>
        <refpurpose>contains the position of the current vertex</refpurpose>
    </refnamediv>
    <refsynopsisdiv>
        <title>Declaration</title>
        <para>
            <varname>gl_Position</varname> is a member of the
            <varname>gl_PerVertex</varname> named block:
<programlisting>    out gl_PerVertex {
        vec4 gl_Position;
        float gl_PointSize;
        float gl_ClipDistance[];
    };</programlisting>
        </para>
    </refsynopsisdiv>
    <refsect1 xml:id="description"><title>Description</title>
        <para>
            In the vertex, tessellation evaluation and geometry languages, a single
            global instance of the <varname>gl_PerVertex</varname> named block is available and
            its <varname>gl_Position</varname> member is an output that receives the
            homogeneous vertex position. It may be written at any time during shader execution.
            The value written to <varname>gl_Position</varname> will be used by primitive assembly,
            clipping, culling and other fixed functionality operations, if present, that operate on
            primitives after vertex processing has occurred.
        </para>
        <para>
            In the tessellation control language, the <varname>gl_PerVertex</varname> named block
            is used to construct an array, <varname>gl_out[]</varname>, whose <varname>gl_Position</varname>
            members hold the homogeneous control point position, which become available as inputs to the subsequent
            tessellation evaluation shader.
        </para>
        <para>
            The value of <varname>gl_Position</varname> (or the <varname>gl_Position</varname> member of the <varname>gl_out[]</varname>
            array, in the case of the tessellation control shader)
            is undefined after the vertex, tessellation control, and tessellation evaluation
            shading stages if the corresponding shader executable does
            not write to gl_Position. It is also undefined after the geometry processing stage if the geometry shader executable calls
            <citerefentry><refentrytitle>EmitVertex</refentrytitle></citerefentry> without having
            written <varname>gl_Position</varname> since the last call to <citerefentry><refentrytitle>EmitVertex</refentrytitle></citerefentry>
            (or hasn't written it at all).
        </para>
        <para>
            In the tessellation control, tessellation evaluation and geometry languages,
            the <varname>gl_PerVertex</varname> named block is used to construct an array, <varname>gl_in[]</varname>
            of per-vertex or per-control point inputs whose content represents the corresponding
            outputs written by the previous stage.
         </para>
    </refsect1>
    <refsect1 xml:id="versions"><title>Version Support</title>
        <informaltable>
            <tgroup cols="11" align="left">
                <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="funchead.xml" xpointer="xpointer(/*/*)"/>
                <tbody>
                    <row>
                        <entry>gl_Position</entry>
                        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="version.xml" xpointer="xpointer(/*/*[@role='11']/*)"/>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
        <para>Versions 1.10 to 1.40 - vertex shader only.</para>
        <para>Versions 1.50 to 3.30 - vertex and geometry shaders only.</para>
    </refsect1>
    <refsect1 xml:id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>gl_PointSize</refentrytitle></citerefentry>, <citerefentry><refentrytitle>gl_ClipDistance</refentrytitle></citerefentry>
        </para>
    </refsect1>
    <refsect1 xml:id="Copyright"><title>Copyright</title>
        <para>
            Copyright <trademark class="copyright"/> 2011-2013 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>
