title: how to apply butterwoth filter on an image
body: <p>I want to apply a <a href="http://www.mathworks.com/help/signal/ref/buttord.html" rel="nofollow">butterworth filter</a> to a photo. The filter design method is <a href="http://www.mathworks.com/help/signal/ref/bilinear.html" rel="nofollow">bilinear</a>.</p>
  
  <p>The filter I designed is:</p>
  
  <pre><code>[N, Wn]=buttord(2, 4.83, -3, -15, 's');
  [b,a]=butter(N,Wn,'s');
  [num,den]=bilinear(b,a,1);
  </code></pre>
  
  <p>How could the filter be applied to a 2D image?</p>