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

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

<div class="refnamediv">

<h2>Name</h2>

<p>gl_FrontFacing — indicates whether a primitive is front or back facing</p>

</div>

<div class="refsynopsisdiv">

<h2>Declaration</h2>

<code class="fieldsynopsis"><span class="modifier">in </span><span class="type">bool </span><span class="varname">gl_FrontFacing </span>;</code>

</div>

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

<h2>Description</h2>

<p>

Available only in the fragment language, <code class="varname">gl_FrontFacing</code> is an input variable

whose value is <code class="function">true</code> 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 <code class="varname">glFrontFace</code>. One way to compute this area is:

</p>

<p>

<math overflow="scroll">

<mi>a</mi><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac>

<msubsup>

<mo>∑</mo>

<mrow>

<mi>j</mi><mo>=</mo><mn>0</mn>

</mrow>

<mrow>

<mi>n</mi><mo>-</mo><mn>1</mn>

</mrow>

</msubsup>

<msubsup>

<mi>x</mi>

<mrow><mi>w</mi></mrow>

<mrow><mi>i</mi></mrow>

</msubsup>

<msubsup>

<mi>y</mi>

<mrow><mi>w</mi></mrow>

<mrow><mi>i</mi><mo>+</mo><mn>1</mn></mrow>

</msubsup>

<mo>-</mo>

<msubsup>

<mi>x</mi>

<mrow><mi>w</mi></mrow>

<mrow><mi>i</mi><mo>+</mo><mn>1</mn></mrow>

</msubsup>

<msubsup>

<mi>y</mi>

<mrow><mi>w</mi></mrow>

<mrow><mi>i</mi></mrow>

</msubsup>

</math>

</p>

<p>

where

<math overflow="scroll">

<msubsup>

<mi>x</mi>

<mrow><mi>w</mi></mrow>

<mrow><mi>i</mi></mrow>

</msubsup>

</math> and

<math overflow="scroll">

<msubsup>

<mi>y</mi>

<mrow><mi>w</mi></mrow>

<mrow><mi>i</mi></mrow>

</msubsup>

</math> are

the x and y window coordinates of the <span class="emphasis"><em>i</em></span>th vertex of the n-vertex polygon.

</p>

<p>

The sign of this computation is negated when the value of

<code class="constant">GL_CLIP_ORIGIN</code> (the clip volume origin, set

with

<a class="citerefentry" href="glClipControl"><span class="citerefentry"><span class="refentrytitle">glClipControl</span></span></a>)

is <code class="constant">GL_UPPER_LEFT</code>.

</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>Variable 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 ; ">gl_FrontFacing</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_FragCoord"><span class="citerefentry"><span class="refentrytitle">gl_FragCoord</span></span></a>,

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