to that region, and
the MATLAB built-in <code/>contour</code>, which draws the contour lines
of a function. We start by</p>

<div class="highlight"><pre><span class="n">M</span> <span class="p">=</span> <span class="nb">zeros</span><span class="p">(</span><span class="nb">size</span><span class="p">(</span><span class="n">I</span><span class="p">))</span> <span class="p">;</span>
<span class="k">for</span> <span class="n">x</span><span class="p">=</span><span class="n">r</span><span class="o">&#39;</span>
 