Image registration with differently coloured backgrounds

Question

I have two images of the same leaf, one on a white background and one on a black. How can I make a program that matches them up as closely as possible (translation + rotation, no scaling)?

The difficulty is the backgrounds - the registration algorithms I've played with have problems because the background is completely different and there is no clean way to convert the images to a common format. The best is extracting the saturation from both images and matching on that, but that still had problems.

Answer 1

Compute the gradient magnitude of both images and register these images by minimizing a normalized cross-correlation or mutual information similarity metric.