1. A method of computing a transformation between a discrete reference signal and a suspect image signal, the method comprising: providing a set of reference signal components representing the discrete reference signal, and providing a transform candidate, wherein the reference signal components, the suspect image signal and the transform candidate are represented as digital, electronic signals in an electronic memory; with digital logic circuitry or programmed processor, applying a transform according to the transform candidate to determine locations of the reference signal components within the suspect image signal; with digital logic circuitry or programmed processor, computing a measure of correlation between the reference signal components and the suspect image signal in the electronic memory when the transform is applied, and based on the measure of correlation, determining whether to update locations of the reference signal components within the suspect image signal; with digital logic circuitry or programmed processor, determining updated locations of the reference signal components within regions around the locations of the reference signal components within the suspect image signal based on values of the suspect image signal in the regions; and with digital logic circuitry or programmed processor, determining a refined linear transform that provides a least squares fit between reference signal component locations and the updated locations.