title: How can I measure the similarity between two images?
body: <p>I would like to compare a screenshot of one application (could be a Web page) with a previously taken screenshot to determine whether the application is displaying itself correctly. I don't want an exact match comparison, because the aspect could be slightly different (in the case of a Web app, depending on the browser, some element could be at a slightly different location). It should give a measure of how similar are the screenshots.</p>
  
  <p>Is there a library / tool that already does that? How would you implement it?</p>