Comparing similar images as photographs -- detecting difference, image diff

Question

The situation is kind of unique from anything I have been able to find asked already, and is as follows: If I took a photo of two similar images, I'd like to be able to highlight the differing features in the two images. For example the following two halves of a children's spot the difference game:

The differences in the images will be bits missing/added and/or colour change, and the type of differences which would be easily detectable from the original image files by doing nothing cleverer than a pixel-by-pixel comparison. However the fact that they're subject to the fluctuations of light and imprecision of photography, I'll need a far more lenient/clever algorithm.

As you can see, the images won't necessarily line up perfectly if overlaid.

This question is tagged language-agnostic as I expect answers that point me towards relevant algorithms, however I'd also be interested in current implementations if they exist, particularly in Java, Ruby, or C.

Answer 1

The following approach should work. All of these functionalities are available in OpenCV. Take a look at this example for computing homographies.

• Detect keypoints in the two images using a corner detector.
• Extract descriptors (SIFT/SURF) for the keypoints.
• Match the keypoints and compute a homography using RANSAC, that aligns the second image to the first.
• Apply the homography to the second image, so that it is aligned with the first.
• Now simply compute the pixel-wise difference between the two images, and the difference image will highlight everything that has changed from the first to the second.