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<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="gl_ClipDistance">
    <info>
        <copyright>
            <year>2011-2013</year>
            <holder>Khronos Group</holder>
        </copyright>
    </info>
    <refmeta>
        <refentrytitle>gl_ClipDistance</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>gl_ClipDistance</refname>
        <refpurpose>provides a forward-compatible mechanism for vertex clipping</refpurpose>
    </refnamediv>
    <refsynopsisdiv>
        <title>Declaration</title>
        <para>
            <varname>gl_ClipDistance</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>
        <para>
            In fragment shaders, it is intrinsically declared as:
            <fieldsynopsis>
                <modifier>in</modifier>
                <type>float</type>
                <varname>gl_ClipDistance[]</varname>
            </fieldsynopsis>
        </para>
    </refsynopsisdiv>
    <refsect1 xml:id="description"><title>Description</title>
        <para>
            The <varname>gl_ClipDistance</varname> variable provides a forward compatible
            mechanism for controlling user clipping. The element <parameter>gl_ClipDistance</parameter>[<emphasis>i</emphasis>]
            specifies a clip distance for each user clip plane <emphasis>i</emphasis>. A distance of 0.0 means that
            the vertex is on the plane, a positive distance means that the vertex is insider the clip plane, and a negative
            distance means that the point is outside the clip plane. The clip distances will be linearly interpolated across
            the primitive and the portion of the primitive with interpolated distances less than 0.0 will be clipped.
        </para>
        <para>
            The <varname>gl_ClipDistance</varname> array is initially predeclared as unsized and must be
            sized by the shader either by redeclaring it with an explicit size, or by indexing it
            with only integral constant expressions. The array must be sized to include all
            clip planes that are enabled via the OpenGL API; if the size does not include all enabled planes,
            results are undefined. The size may be at most <varname>gl_MaxClipDistances</varname>. The number
            of varying components consumed by <varname>gl_ClipDistance</varname> will match the size of the
            array, no matter how many planes are enabled. The shader must also set all values in <varname>gl_ClipDistance</varname>
            that have been enabled via the OpenGL API, or results are undefined. Values written into
            <varname>gl_ClipDistance</varname> planes that are not enabled have no effect.
        </para>
        <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_ClipDistance</varname> member is an output that receives the
            clip distances for the current vertex. It may be written at any time during shader execution.
            The value written to <varname>gl_ClipDistance</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_ClipDistance</varname>
            members hold clip distances for each of the control points, which become available as inputs to the subsequent
            tessellation evaluation shader.
        </para>
        <para>
            The value of <varname>gl_ClipDistance</varname> (or the <varname>gl_ClipDistance</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_ClipDistance. 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_ClipDistance</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 geoemetry 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. Only elements of the <varname>gl_ClipDistance</varname>
            array that correspond to enabled clip planes have defined values.
         </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="varhead.xml" xpointer="xpointer(/*/*)"/>
                <tbody>
                    <row>
                        <entry>gl_ClipDistance</entry>
                        <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="version.xml" xpointer="xpointer(/*/*[@role='13']/*)"/>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
        <para>Versions 1.30 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>
        </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>
