
<refentry id="gluPickMatrix">
    <refmeta>
        <refmetainfo>
            <copyright>
                <year>1991-2006</year>
                <holder>Silicon Graphics, Inc.</holder>
            </copyright>
        </refmetainfo>
        <refentrytitle>gluPickMatrix</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>gluPickMatrix</refname>
        <refpurpose>define a picking region</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void <function>gluPickMatrix</function></funcdef>
                <paramdef>GLdouble <parameter>x</parameter></paramdef>
                <paramdef>GLdouble <parameter>y</parameter></paramdef>
                <paramdef>GLdouble <parameter>delX</parameter></paramdef>
                <paramdef>GLdouble <parameter>delY</parameter></paramdef>
                <paramdef>GLint * <parameter>viewport</parameter></paramdef>
            </funcprototype>
        </funcsynopsis>
    </refsynopsisdiv>
    <!-- eqn: ignoring delim $$ -->
    <refsect1 id="parameters"><title>Parameters</title>
        <variablelist>
        <varlistentry>
            <term><parameter>x</parameter></term>
            <term><parameter>y</parameter></term>
            <listitem>
                <para>
                    Specify the center of a picking region in window coordinates.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>delX</parameter></term>
            <term><parameter>delY</parameter></term>
            <listitem>
                <para>
                    Specify the width and height, respectively, of the picking region in window 
                    coordinates.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>viewport</parameter></term>
            <listitem>
                <para>
                    Specifies the current viewport (as from a <citerefentry><refentrytitle>glGetIntegerv</refentrytitle></citerefentry> call).
                </para>
            </listitem>
        </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 id="description"><title>Description</title>
        <para>
            <function>gluPickMatrix</function> creates a projection matrix that can be used to restrict drawing
            to a small region of the viewport.
            This is typically useful to
            determine what objects are being drawn near the cursor.
            Use <function>gluPickMatrix</function> to
            restrict drawing to a small region around the cursor.
            Then,
            enter selection mode (with <citerefentry><refentrytitle>glRenderMode</refentrytitle></citerefentry>) and rerender the scene.
            All primitives that would have been drawn near
            the cursor are identified and stored in the selection buffer.
        </para>
        <para>
            The matrix created by <function>gluPickMatrix</function> is multiplied by the current matrix just
            as if <citerefentry><refentrytitle>glMultMatrix</refentrytitle></citerefentry> is called with the generated matrix.
            To effectively use the generated pick matrix for picking,
            first call <citerefentry><refentrytitle>glLoadIdentity</refentrytitle></citerefentry> to load an identity matrix onto the
            perspective matrix stack.
            Then call <function>gluPickMatrix</function>,
            and, finally, call a command (such as <citerefentry><refentrytitle>gluPerspective</refentrytitle></citerefentry>)
            to multiply the perspective matrix by the pick matrix.
        </para>
        <para>
            When using <function>gluPickMatrix</function> to pick NURBS, be careful to turn off the NURBS 
            property
            <constant>GLU_AUTO_LOAD_MATRIX</constant>.  If <constant>GLU_AUTO_LOAD_MATRIX</constant> is not
            turned off, then any NURBS surface rendered is subdivided differently with
            the pick matrix than the way it was subdivided without the pick matrix.
        </para>
    </refsect1>
    <refsect1 id="example"><title>Example</title>
        <para>
            When rendering a scene as follows:
            <programlisting>
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(...);
glMatrixMode(GL_MODELVIEW);
/* Draw the scene */
            </programlisting>
            a portion of the viewport can be selected as a pick region like this:
            <programlisting>
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPickMatrix(x, y, width, height, viewport);
gluPerspective(...);
glMatrixMode(GL_MODELVIEW);
/* Draw the scene */
            </programlisting>
        </para>
    </refsect1>
    <refsect1 id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>gluPerspective</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glLoadIdentity</refentrytitle></citerefentry>, 
            <citerefentry><refentrytitle>glMultMatrix</refentrytitle></citerefentry>, 
            <citerefentry><refentrytitle>glRenderMode</refentrytitle></citerefentry>
        </para>
    </refsect1>
    <refsect1 id="Copyright"><title>Copyright</title>
        <para>
            Copyright <trademark class="copyright"></trademark> 1991-2006
            Silicon Graphics, Inc. This document is licensed under the SGI
            Free Software B License. For details, see
            <ulink url="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</ulink>.
        </para>
    </refsect1>
</refentry>
