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<refentry id="glDrawElementsInstancedBaseInstance">
    <refentryinfo>
        <copyright>
            <year>2011-2013</year>
            <holder>Khronos Group</holder>
        </copyright>
    </refentryinfo>
    <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 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 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><![CDATA[    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>
                  <mml:mrow>
                    <mml:mfenced open = '&lfloor;' close = '&rfloor;'>
                      <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'>&plus;</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="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 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 id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>glDrawElements</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glDrawArraysInstanced</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>
