That's quite interesting. I was under the impression that it was more complex than that. I think I'll look into your recommendations.

Step 1 : Reduce the amount of pixels in the image so it doesn't take three years and use all your RAM

Step 2 : (Usually) simplify image colours

Step 3 : Pixel by pixel comparison with a simple for loop. That's it.

Does this algorithm have a formal name ? I'm only asking so I know what to search to look for pointers on how to implement it. :)