ce between patches -- NOT normalized correlation since we assume
 pixel brightness constancy.  Once the closest matching patch is found,
 the alignment between the two patches is further refined using lucas &amp;
 kanade to find the subpixel translation vector relating the two patches.

 This code has been refined for speed, but since it is nonvectorized code
 it can be fairly slow.  Running time is linear in the number of pixels
 but the constant is fairly large.  Test on small image (150x150) befor