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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="glDrawElementsInstancedBaseInstance">
<info>
<copyright>
<year>2011-2014</year>
<holder>Khronos Group</holder>
</copyright>
</info>
<refmeta>
<refentrytitle>glDrawElementsInstancedBaseInstance</refentrytitle>
<manvolnum>3G</manvolnum>
</refmeta>
<refnamediv>
<refname>glDrawElementsInstancedBaseInstance</refname>
<refpurpose>draw multiple instances of a set of elements with offset applied to instanced attributes</refpurpose>
</refnamediv>
<refsynopsisdiv><title>C Specification</title>
<funcsynopsis>
<funcprototype>
<funcdef>void <function>glDrawElementsInstancedBaseInstance</function></funcdef>
<paramdef>GLenum <parameter>mode</parameter></paramdef>
<paramdef>GLsizei <parameter>count</parameter></paramdef>
<paramdef>GLenum <parameter>type</parameter></paramdef>
<paramdef>const void * <parameter>indices</parameter></paramdef>
<paramdef>GLsizei <parameter>primcount</parameter></paramdef>
<paramdef>GLuitn <parameter>baseinstance</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1 xml:id="parameters"><title>Parameters</title>
<variablelist>
<varlistentry>
<term><parameter>mode</parameter></term>
<listitem>
<para>
Specifies what kind of primitives to render.
Symbolic constants
<constant>GL_POINTS</constant>,
<constant>GL_LINE_STRIP</constant>,
<constant>GL_LINE_LOOP</constant>,
<constant>GL_LINES</constant>,
<constant>GL_LINE_STRIP_ADJACENCY</constant>,
<constant>GL_LINES_ADJACENCY</constant>,
<constant>GL_TRIANGLE_STRIP</constant>,
<constant>GL_TRIANGLE_FAN</constant>,
<constant>GL_TRIANGLES</constant>,
<constant>GL_TRIANGLE_STRIP_ADJACENCY</constant>,
<constant>GL_TRIANGLES_ADJACENCY</constant> and <constant>GL_PATCHES</constant>
are accepted.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>count</parameter></term>
<listitem>
<para>
Specifies the number of elements to be rendered.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>type</parameter></term>
<listitem>
<para>
Specifies the type of the values in <parameter>indices</parameter>. Must be one of <constant>GL_UNSIGNED_BYTE</constant>,
<constant>GL_UNSIGNED_SHORT</constant>, or <constant>GL_UNSIGNED_INT</constant>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>indices</parameter></term>
<listitem>
<para>
Specifies a pointer to the location where the indices are stored.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>primcount</parameter></term>
<listitem>
<para>
Specifies the number of instances of the specified range of indices to be rendered.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>baseinstance</parameter></term>
<listitem>
<para>
Specifies the base instance for use in fetching instanced vertex attributes.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 xml:id="description"><title>Description</title>
<para>
<function>glDrawElementsInstancedBaseInstance</function> behaves identically to <citerefentry><refentrytitle>glDrawElements</refentrytitle></citerefentry>
except that <parameter>primcount</parameter> instances of the set of elements are executed and the value of the internal counter
<parameter>instanceID</parameter> advances for each iteration. <parameter>instanceID</parameter> is an internal 32-bit integer counter
that may be read by a vertex shader as <constant>gl_InstanceID</constant>.
</para>
<para>
<function>glDrawElementsInstancedBaseInstance</function> has the same effect as:
<programlisting> if (mode, count, or type is invalid )
generate appropriate error
else {
for (int i = 0; i < primcount ; i++) {
instanceID = i;
glDrawElements(mode, count, type, indices);
}
instanceID = 0;
}</programlisting>
</para>
<para>
Specific vertex attributes may be classified as <emphasis>instanced</emphasis> through the use of
<citerefentry><refentrytitle>glVertexAttribDivisor</refentrytitle></citerefentry>. Instanced vertex attributes
supply per-instance vertex data to the vertex shader. The index of the vertex fetched from the enabled instanced vertex
attribute arrays is calculated as
<inlineequation>
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
<mml:mrow>
<mml:mfenced open="⌊" close="⌋">
<mml:mfrac>
<mml:mrow>
<mml:mi>gl</mml:mi>
<mml:mo>_</mml:mo>
<mml:mi>InstanceID</mml:mi>
</mml:mrow>
<mml:mi>divisor</mml:mi>
</mml:mfrac>
</mml:mfenced>
<mml:mo lspace="2px" rspace="2px">+</mml:mo>
<mml:mi>baseInstance</mml:mi>
</mml:mrow>
</mml:math>
</inlineequation>. Note that
<parameter>baseinstance</parameter> does not affect the
shader-visible value of <constant>gl_InstanceID</constant>.
</para>
</refsect1>
<refsect1 xml:id="notes"><title>Notes</title>
<para>
<function>glDrawElementsInstancedBaseInstance</function> is available only if the GL version is 4.2 or greater.
</para>
<para>
<constant>GL_LINE_STRIP_ADJACENCY</constant>,
<constant>GL_LINES_ADJACENCY</constant>,
<constant>GL_TRIANGLE_STRIP_ADJACENCY</constant> and
<constant>GL_TRIANGLES_ADJACENCY</constant>
are available only if the GL version is 3.2 or greater.
</para>
</refsect1>
<refsect1 xml:id="errors"><title>Errors</title>
<para>
<constant>GL_INVALID_ENUM</constant> is generated if <parameter>mode</parameter> is not one of <constant>GL_POINTS</constant>,
<constant>GL_LINE_STRIP</constant>, <constant>GL_LINE_LOOP</constant>, <constant>GL_LINES</constant>,
<constant>GL_TRIANGLE_STRIP</constant>, <constant>GL_TRIANGLE_FAN</constant>, or <constant>GL_TRIANGLES</constant>.
</para>
<para>
<constant>GL_INVALID_VALUE</constant> is generated if <parameter>count</parameter> or <parameter>primcount</parameter> are negative.
</para>
<para>
<constant>GL_INVALID_OPERATION</constant> is generated if a geometry shader is active and <parameter>mode</parameter>
is incompatible with the input primitive type of the geometry shader in the currently installed program object.
</para>
<para>
<constant>GL_INVALID_OPERATION</constant> is generated if a non-zero buffer object name is bound to an
enabled array and the buffer object's data store is currently mapped.
</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="apifunchead.xml" xpointer="xpointer(/*/*)"/>
<tbody>
<row>
<entry><function>glDrawElementsInstancedBaseInstance</function></entry>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="apiversion.xml" xpointer="xpointer(/*/*[@role='42']/*)"/>
</row>
</tbody>
</tgroup>
</informaltable>
</refsect1>
<refsect1 xml:id="seealso"><title>See Also</title>
<para>
<citerefentry><refentrytitle>glDrawElements</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glDrawArraysInstanced</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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