<p>The stability of an extremal region <code/>R</code> is the inverse
of the relative area variation of the region <code/>R</code> when the
intensity level is increased by <code/>&Delta;</code>. Formally, the
variation is defined as:</p>

<div class="highlight"><pre><span class="o">|</span><span class="n">R</span><span class="p">(</span><span class="o">+</span>Î”<span class="p">)</span> <span class="o">-</span> <span class="n">R</span><span class="o">|</span>
<span class="o">-----------</span>
    <span 