1. A method of determining a coordinate transform between a first image and a second image from a selected subset of candidate alignment patches, said method comprising the steps of: determining a rate of change of pixel values for each of a plurality of locations on the first image to identify a set of candidate alignment patches in the first image; specifying a plurality of subsets of candidate alignment patches from the set of candidate alignment patches based on an error metric corresponding to each subset, said error metric depending upon said rate of change of pixel values for the patches in a corresponding subset; selecting a subset of candidate alignment patches from said plurality of subsets of candidate alignment patches based upon a predetermined criterion; estimating, for each patch in the selected subset, a shift between the patch and a corresponding patch in the second image, the location of the corresponding patch in the second image being determined from the location of the patch in the first image; and determining the coordinate transform between the first image and second image based on the estimated shifts.