title: Find duplicate images of different sizes
body: <p>I am wondering if there is a pre-existing algorithm/library/framework to compare two images to see if one is a re-sized version of the other? The programming language doesn't matter at this stage. </p>
  
  <p>If there is nothing out there, I'd need to write something up. What I have thought of so far:</p>
  
  <ul>
  <li><p>(Expensive) Resize the larger to the smaller and compare pixel by pixel. </p></li>
  <li><p>Better yet, just resize a few random "areas" on the picture and compare. If they match, convert more, etc... </p></li>
  <li><p>Break the image into a number of rows and columns and do some sort of parity math on the color values. </p></li>
  </ul>
  
  <p>The problem I see with the first two ideas especially, is that there are different ways to re-size a picture in the first place, so the math will likely not work out the same at all. Some re-sizing adds blur, etc....</p>
  
  <p>If anyone could point me to some good literature on this subject, that would be great. My googling turns up mostly shareware applications which is not what I want. </p>
  
  <p>The goal is to have this running in the back of a webserver. </p>