an class="o">~</span> <span class="n">exist</span><span class="p">(</span><span class="n">pfx</span><span class="p">,</span> <span class="s">&#39;dir&#39;</span><span class="p">)</span>
  <span class="n">mkdir</span><span class="p">(</span><span class="n">pfx</span><span class="p">)</span> <span class="p">;</span>
<span class="k">end</span>

<span class="n">p_img</span> <span class="p">=</span> <span class="n">fullfile</span><span class="p">(</span><span class="n">vl_root</span><span class="p">,</span><span