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<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="glBeginTransformFeedback">
    <info>
        <copyright>
            <year>2010-2013</year>
            <holder>Khronos Group</holder>
        </copyright>
    </info>
    <refmeta>
        <refentrytitle>glBeginTransformFeedback</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>glBeginTransformFeedback</refname>
        <refpurpose>start transform feedback operation</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void <function>glBeginTransformFeedback</function></funcdef>
                <paramdef>GLenum <parameter>primitiveMode</parameter></paramdef>
            </funcprototype>
        </funcsynopsis>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void <function>glEndTransformFeedback</function></funcdef>
                <paramdef><parameter>void</parameter></paramdef>
            </funcprototype>
        </funcsynopsis>
    </refsynopsisdiv>
    <refsect1 xml:id="parameters"><title>Parameters for <function>glBeginTransformFeedback</function></title>
        <variablelist>
        <varlistentry>
            <term><parameter>primitiveMode</parameter></term>
            <listitem>
                <para>
                    Specify the output type of the primitives that will be recorded into the
                    buffer objects that are bound for transform feedback.
                </para>
            </listitem>
        </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 xml:id="description"><title>Description</title>
        <para>
            Transform feedback mode captures the values of varying variables written by the vertex shader (or, if active, the geometry shader).
            Transform feedback is said to be active after a call to <function>glBeginTransformFeedback</function>
            until a subsequent call to <citerefentry><refentrytitle>glEndTransformFeedback</refentrytitle></citerefentry>.
            Transform feedback commands must be paired.
        </para>
        <para>
            If no geometry shader is present, while transform feedback is active the <parameter>mode</parameter> parameter to
            <citerefentry><refentrytitle>glDrawArrays</refentrytitle></citerefentry> must match those specified
            in the following table:
        </para>
        <informaltable>
            <tgroup cols="2" align="left">
                <colspec colwidth="1.1*"/>
                <colspec colwidth="1*"/>
                <thead>
                    <row>
                        <entry>
                            <emphasis role="bold"> 
                                Transform Feedback <parameter>primitiveMode</parameter>
                            </emphasis>
                        </entry> 
                        <entry>
                            <emphasis role="bold"> 
                                Allowed Render Primitive <parameter>modes</parameter>
                            </emphasis>
                        </entry>
                    </row>
                </thead>
                <tbody>
                    <row>
                        <entry> <constant>GL_POINTS</constant> </entry>
                        <entry> <constant>GL_POINTS</constant> </entry>
                    </row>
                    <row>
                        <entry> <constant>GL_LINES</constant> </entry>
                        <entry>
                            <constant>GL_LINES</constant>,
                            <constant>GL_LINE_LOOP</constant>,
                            <constant>GL_LINE_STRIP</constant>,
                            <constant>GL_LINES_ADJACENCY</constant>,
                            <constant>GL_LINE_STRIP_ADJACENCY</constant>
                        </entry>
                    </row>
                    <row>
                        <entry>
                            <constant>GL_TRIANGLES</constant>
                        </entry>
                        <entry>
                            <constant>GL_TRIANGLES</constant>, 
                            <constant>GL_TRIANGLE_STRIP</constant>, 
                            <constant>GL_TRIANGLE_FAN</constant>,
                            <constant>GL_TRIANGLES_ADJACENCY</constant>, 
                            <constant>GL_TRIANGLE_STRIP_ADJACENCY</constant>
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
        <para>
            If a geometry shader is present, the output primitive type from the geometry shader must match those
            provided in the following table:
        </para>
        <informaltable>
            <tgroup cols="2" align="left">
                <colspec colwidth="1.1*"/>
                <colspec colwidth="1*"/>
                <thead>
                    <row>
                        <entry>
                            <emphasis role="bold">
                                Transform Feedback <parameter>primitiveMode</parameter>
                            </emphasis>
                        </entry>
                        <entry>
                            <emphasis role="bold">
                                Allowed Geometry Shader Output Primitive Type
                            </emphasis>
                        </entry>
                    </row>
                </thead>
                <tbody>
                    <row>
                        <entry> <constant>GL_POINTS</constant> </entry>
                        <entry> <constant>points</constant> </entry>
                    </row>
                    <row>
                        <entry> <constant>GL_LINES</constant> </entry>
                        <entry> <constant>line_strip</constant> </entry>
                    </row>
                    <row>
                        <entry> <constant>GL_TRIANGLES</constant> </entry>
                        <entry> <constant>triangle_strip</constant> </entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
    </refsect1>
    <refsect1 xml:id="notes"><title>Notes</title>
        <para>
            Geometry shaders, and the <constant>GL_TRIANGLES_ADJACENCY</constant>, <constant>GL_TRIANGLE_STRIP_ADJACENCY</constant>,
            <constant>GL_LINES_ADJACENCY</constant> and <constant>GL_LINE_STRIP_ADJACENCY</constant> primtive modes 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_OPERATION</constant> is generated if <function>glBeginTransformFeedback</function> is executed
            while transform feedback is active.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if <function>glEndTransformFeedback</function> is executed
            while transform feedback is not active.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated by <citerefentry><refentrytitle>glDrawArrays</refentrytitle></citerefentry>
            if no geometry shader is present, transform feedback is active and <parameter>mode</parameter> is not one of the allowed modes.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated by <citerefentry><refentrytitle>glDrawArrays</refentrytitle></citerefentry>
            if a geometry shader is present, transform feedback is active and the output primitive type of the geometry shader does not
            match the transform feedback <parameter>primitiveMode</parameter>.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated by <function>glEndTransformFeedback</function> if any binding
            point used in transform feedback mode does not have a buffer object bound.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated by <function>glEndTransformFeedback</function> if no binding
            points would be used, either because no program object is active of because the active program object has specified
            no varying variables to record.
        </para>
    </refsect1>
    <refsect1 xml:id="Copyright"><title>Copyright</title>
        <para>
            Copyright <trademark class="copyright"/> 2010-2013 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.
            <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://opencontent.org/openpub/">http://opencontent.org/openpub/</link>.
        </para>
    </refsect1>
</refentry>
