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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
"http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
<refentry id="glDrawArraysInstancedBaseInstance">
<refentryinfo>
<copyright>
<year>2011-2013</year>
<holder>Khronos Group</holder>
</copyright>
</refentryinfo>
<refmeta>
<refentrytitle>glDrawArraysInstancedBaseInstance</refentrytitle>
<manvolnum>3G</manvolnum>
</refmeta>
<refnamediv>
<refname>glDrawArraysInstancedBaseInstance</refname>
<refpurpose>draw multiple instances of a range of elements with offset applied to instanced attributes</refpurpose>
</refnamediv>
<refsynopsisdiv><title>C Specification</title>
<funcsynopsis>
<funcprototype>
<funcdef>void <function>glDrawArraysInstancedBaseInstance</function></funcdef>
<paramdef>GLenum <parameter>mode</parameter></paramdef>
<paramdef>GLint <parameter>first</parameter></paramdef>
<paramdef>GLsizei <parameter>count</parameter></paramdef>
<paramdef>GLsizei <parameter>primcount</parameter></paramdef>
<paramdef>GLuint <parameter>baseinstance</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1 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_TRIANGLE_STRIP</constant>, <constant>GL_TRIANGLE_FAN</constant>, <constant>GL_TRIANGLES</constant>
<constant>GL_LINES_ADJACENCY</constant>, <constant>GL_LINE_STRIP_ADJACENCY</constant>,
<constant>GL_TRIANGLES_ADJACENCY</constant>, <constant>GL_TRIANGLE_STRIP_ADJACENCY</constant> and <constant>GL_PATCHES</constant>
are accepted.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>first</parameter></term>
<listitem>
<para>
Specifies the starting index in the enabled arrays.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>count</parameter></term>
<listitem>
<para>
Specifies the number of indices to be rendered.
</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 id="description"><title>Description</title>
<para>
<function>glDrawArraysInstancedBaseInstance</function> behaves identically to <citerefentry><refentrytitle>glDrawArrays</refentrytitle></citerefentry>
except that <parameter>primcount</parameter> instances of the range 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>glDrawArraysInstancedBaseInstance</function> has the same effect as:
<programlisting> if ( mode or count is invalid )
generate appropriate error
else {
for (int i = 0; i < primcount ; i++) {
instanceID = i;
glDrawArrays(mode, first, count);
}
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>
<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 id="errors"><title>Errors</title>
<para>
<constant>GL_INVALID_ENUM</constant> is generated if <parameter>mode</parameter> is not one of
the accepted values.
</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_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 non-zero buffer object name is bound to an
enabled array and the buffer object's data store is currently mapped.
</para>
</refsect1>
<refsect1 id="seealso"><title>See Also</title>
<para>
<citerefentry><refentrytitle>glDrawArrays</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glDrawElementsInstanced</refentrytitle></citerefentry>
</para>
</refsect1>
<refsect1 id="Copyright"><title>Copyright</title>
<para>
Copyright <trademark class="copyright"></trademark> 2011-2013 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.
<ulink url="http://opencontent.org/openpub/">http://opencontent.org/openpub/</ulink>.
</para>
</refsect1>
</refentry>
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