title: show only difference using imagemagick compare
body: <p>I'm using ImageMagick compare to compare two images and get their diff. I have 2 questions:</p>
  
  <ol>
  <li>I want to get a transparent result image with only the diff parts, nothing else. Not white - transparent.</li>
  <li>Is it possible to get the diff of only one image instead of both ?</li>
  </ol>
  
  <p>Thanks!</p>