<div class="refentry" id="gl_ViewportIndex">

<div class="titlepage"></div>

<div class="refnamediv">

<h2>Name</h2>

<p>gl_ViewportIndex — contains the index of the viewport to be used in viewport transformation and scissoring</p>

</div>

<div class="refsynopsisdiv">

<h2>Declaration</h2>

<p>

</p>

<pre class="programlisting">// In geometry shaders</pre>

<p>

<code class="fieldsynopsis"><span class="modifier">out </span><span class="type">int </span><span class="varname">gl_ViewportIndex </span>;</code>

</p>

<p>

</p>

<pre class="programlisting">// In fragment shaders</pre>

<p>

<code class="fieldsynopsis"><span class="modifier">in </span><span class="type">int </span><span class="varname">gl_ViewportIndex </span>;</code>

</p>

</div>

<div class="refsect1" id="description">

<h2>Description</h2>

<p>

In the geometry language, <code class="varname">gl_ViewportIndex</code> is used to specify the index

of the viewport to which the next primitive emitted from the shader should be drawn. Primitives generated

by the geometry shader will undergo viewport transformation and scissor testing using the viewport

transformation and scissor rectangle selected by the value of <code class="varname">gl_ViewportIndex</code>.

The viewport index used will come from one of the vertices in the primitive being shaded. However, which

vertex the viewport index comes from is implementation-defined, and so it is recommended to use the same

viewport index for all vertices of a primitive. If a geometry shader does not assign a value to

<code class="varname">gl_ViewportIndex</code>, viewport transform and scissor rectangle zero will be used.

If a geometry shader statically assigns a value to <code class="varname">gl_ViewportIndex</code> and there

is a path through the shader that does not assign a value to <code class="varname">gl_ViewportIndex</code>,

the value of <code class="varname">gl_ViewportIndex</code> is undefined for executions of the shader that take

that path.

</p>

<p>

In the fragment language, <code class="varname">gl_ViewportIndex</code> will have the same value that was written to the

output variable <code class="varname">gl_ViewportIndex</code> in the geometry stage. If the geometry stage does not dynamically

assign to <code class="varname">gl_ViewportIndex</code>, the value of <code class="varname">gl_ViewportIndex</code> in the fragment shader will be undefined. If

the geometry stage makes no static assignment to <code class="varname">gl_ViewportIndex</code>, the fragment stage will read zero.

Otherwise, the fragment stage will read the same value written by the geometry stage, even if that value is

out of range. If a fragment shader contains a static access to <code class="varname">gl_ViewportIndex</code>, it will count against the

implementation defined limit for the maximum number of inputs to the fragment stage.

</p>

</div>

{$pipelinestall}{$examples}

<div class="refsect1" id="versions">

<h2>Version Support</h2>

<div class="informaltable">

<table style="border-collapse: collapse; border-top: 2px solid ; border-bottom: 2px solid ; border-left: 2px solid ; border-right: 2px solid ; ">

<colgroup>

<col style="text-align: left; "/>

<col style="text-align: center; " class="firstvers"/>

<col style="text-align: center; "/>

<col style="text-align: center; "/>

<col style="text-align: center; "/>

<col style="text-align: center; "/>

<col style="text-align: center; "/>

<col style="text-align: center; "/>

<col style="text-align: center; "/>

<col style="text-align: center; "/>

<col style="text-align: center; "/>

<col style="text-align: center; "/>

<col style="text-align: center; " class="lastvers"/>

</colgroup>

<thead>

<tr>

<th style="text-align: left; border-right: 2px solid ; ">

</th>

<th style="text-align: center; border-bottom: 2px solid ; " colspan="12">

<span class="bold"><strong>OpenGL Shading Language Version</strong></span>

</th>

</tr>

<tr>

<th style="text-align: left; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>Function Name</strong></span>

</th>

<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>1.10</strong></span>

</th>

<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>1.20</strong></span>

</th>

<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>1.30</strong></span>

</th>

<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>1.40</strong></span>

</th>

<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>1.50</strong></span>

</th>

<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>3.30</strong></span>

</th>

<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>4.00</strong></span>

</th>

<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>4.10</strong></span>

</th>

<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>4.20</strong></span>

</th>

<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>4.30</strong></span>

</th>

<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">

<span class="bold"><strong>4.40</strong></span>

</th>

<th style="text-align: center; border-bottom: 2px solid ; ">

<span class="bold"><strong>4.50</strong></span>

</th>

</tr>

</thead>

<tbody>

<tr>

<td style="text-align: left; border-right: 2px solid ; border-bottom: 2px solid ; ">gl_ViewportIndex (geometry stage)</td>

<td style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">✔</td>

<td style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">✔</td>

<td style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">✔</td>

<td style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">✔</td>

<td style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">✔</td>

<td style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">✔</td>

<td style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">✔</td>

<td style="text-align: center; border-bottom: 2px solid ; ">✔</td>

</tr>

<tr>

<td style="text-align: left; border-right: 2px solid ; ">gl_ViewportIndex (fragment stage)</td>

<td style="text-align: center; border-right: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; ">-</td>

<td style="text-align: center; border-right: 2px solid ; ">✔</td>

<td style="text-align: center; border-right: 2px solid ; ">✔</td>

<td style="text-align: center; ">✔</td>

</tr>

</tbody>

</table>

</div>

</div>

<div class="refsect1" id="seealso">

<h2>See Also</h2>

<p>

<a class="citerefentry" href="gl_Layer"><span class="citerefentry"><span class="refentrytitle">gl_Layer</span></span></a>

</p>

</div>

<div class="refsect1" id="Copyright">

<h2>Copyright</h2>

<p>

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.

<a class="link" href="https://opencontent.org/openpub/" target="_top">https://opencontent.org/openpub/</a>.

</p>

</div>

</div>