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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_Position">
<info>
<copyright>
<year>2011-2014</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="13" 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"/> 2011-2014 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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