Image registration with differently coloured backgrounds

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 backgr开发者_JAVA百科ounds - 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.

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