title: Compare Scanned Images and detect very small visible changes
body: <p>I need a solution to compare two scanned images.
  I have an image of an application form (unfilled), I need to compare that against other images of the same form, and want to detect whether there is any totally unfilled application form.</p>
  
  <p>I just tried with Emgu CV AbsDiff, MatchTemplate etc, but none of them give me a 100 % match, even if I scanned the same form twice in the same scanner, could be because of the noise in the scanning, I can apply a tolerance but the problem is that I need to find out whether the user has filled anything in it. If I apply a tolerance then small changes in the form will not be detected.</p>
  
  <p>I also had a look at the Python Image Libray, Accord.Net etc but couldn't find an approach for comparing this type of image.</p>
  
  <p>Any suggestions on how to do this type of image comparison ?
  Is there any free or paid library available for this ?</p>