image on the plane of two transformed variables
that shows the categories of pixels most clearly (<code><a href='plot_rgb_plane.html'>plot_rgb_plane()</a></code>,
<code>plot_pixels</code>).</p></li>
<li><p>trace lines between the pixel clusters and use them to create
classification rules (<code><a href='place_rule.html'>place_rule()</a></code>, <code>define_rule</code>,
<code><a href='plot_rule.html'>plot_rule()</a></code>).</p></li>
<li><p>combine the rules to define categories. Sometimes the rules are
com