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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="noise">
<info>
<copyright>
<year>2011-2014</year>
<holder>Khronos Group</holder>
</copyright>
</info>
<refmeta>
<refentrytitle>noise</refentrytitle>
<manvolnum>3G</manvolnum>
</refmeta>
<refnamediv>
<refname>noise</refname>
<refpurpose>generate values with a pseudo-random noise function</refpurpose>
</refnamediv>
<refsynopsisdiv><title>Declaration</title>
<funcsynopsis>
<funcprototype>
<funcdef>float <function>noise1</function></funcdef>
<paramdef>genType <parameter>x</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>vec2 <function>noise2</function></funcdef>
<paramdef>genType <parameter>x</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>vec3 <function>noise3</function></funcdef>
<paramdef>genType <parameter>x</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>vec4 <function>noise4</function></funcdef>
<paramdef>genType <parameter>x</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1 xml:id="parameters"><title>Parameters</title>
<variablelist>
<varlistentry>
<term><parameter>x</parameter></term>
<listitem>
<para>
Specifies the value to be used to seed the noise function.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 xml:id="description"><title>Description</title>
<para>
<function>noise1</function>, <function>noise2</function>, <function>noise3</function> and <function>noise4</function> return
noise values (vector or scalar) based on the input value <parameter>x</parameter>. The noise function is a stochastic function
that can be used to increase visual complexity. Values returned by the noise functions give the appearance of randomness,
but are not truly random. They are defined to have the following characteristics:
</para>
<para>
<itemizedlist>
<listitem><para>The return value(s) are always in the range [-1.0,1.0], and cover at least the range [-0.6, 0.6], with a Gaussian-like distribution.</para></listitem>
<listitem><para>The return value(s) have an overall average of 0.0.</para></listitem>
<listitem><para>They are repeatable, in that a particular input value will always produce the same return value.</para></listitem>
<listitem><para>They are statistically invariant under rotation (i.e., no matter how the domain is rotated, it has the same statistical character).</para></listitem>
<listitem><para>They have a statistical invariance under translation (i.e., no matter how the domain is translated, it has the same statistical character).</para></listitem>
<listitem><para>They typically give different results under translation.</para></listitem>
<listitem><para>The spatial frequency is narrowly concentrated, centered somewhere between 0.5 to 1.0.</para></listitem>
<listitem><para>They are C1 continuous everywhere (i.e., the first derivative is continuous).</para></listitem>
</itemizedlist>
</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>noise1</entry>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="version.xml" xpointer="xpointer(/*/*[@role='11']/*)"/>
</row>
<row>
<entry>noise2</entry>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="version.xml" xpointer="xpointer(/*/*[@role='11']/*)"/>
</row>
<row>
<entry>noise3</entry>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="version.xml" xpointer="xpointer(/*/*[@role='11']/*)"/>
</row>
<row>
<entry>noise4</entry>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="version.xml" xpointer="xpointer(/*/*[@role='11']/*)"/>
</row>
</tbody>
</tgroup>
</informaltable>
</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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