1. A method of registering images, the method comprising: for each image, determining an optimum intensity threshold set from a plurality of intensity threshold sets that maximizes a variance between classes of each set using a computer; segmenting each image using the corresponding determined optimum intensity threshold set using the computer; generating a joint histogram of at least two of the segmented images using the computer; generating mutual information from the joint histogram using the computer; and registering the at least two images based on the mutual information using the computer, wherein the joint histogram is generated by a geometry shader of a graphical processing unit GPU of the computer in a texture object attached to a frame buffer of the GPU.