title: 'cannot filter palette images' error when doing a ImageEnhance.Sharpness()
body: <p>I have a GIF image file. I opened it using <a href="http://effbot.org/imagingbook/image.htm" rel="nofollow"><code>PIL.Image</code></a> and did a couple of size transforms on it. Then I tried to use <a href="http://effbot.org/imagingbook/imageenhance.htm#tag-ImageEnhance.Sharpness" rel="nofollow"><code>ImageSharpness.Enhance()</code></a> on it...</p>
  
  <pre><code>sharpener = PIL.ImageEnhance.Sharpness(img)
  sharpened = sharpener.enhance(2.0)
  </code></pre>
  
  <p>This is causing an exception:</p>
  
  <pre><code>&lt;type 'exceptions.ValueError'&gt;
  ('cannot filter palette images',)
  </code></pre>
  
  <p>I tried to google for this error, but did not find anything. Can someone help me figure out what is going wrong?</p>
  
  <p>FYI the mode of the input image is <code>'P'</code>. I don't have this problem if I work with jpg images.</p>