ased by <code>&Delta;</code>. Formally, the
variation is defined as:</p>

<precode type='matlab'>
|R(+&Delta;) - R|
-----------
    |R|
</precode>

<p>where <code>|R|</code> denotes the area of the extremal region
<code>R</code>, <code>R(+&Delta;)</code> is the extremal region
<code>+&Delta;</code> levels up which contains <code>R</code> and
<code>|R(+&Delta;) - R|</code> is the area difference of the two
regions. </p>

<p>A stable region has a small variation. The algorithm finds regions which
are "maximal