1. A method for comparing a model image with a run-time image so as to provide a quantitative measure of image similarity, the method comprising: at train-time, extracting sample pixels of the model image so as to provide a pixel position and a gray value for each sample pixel of the model image; computing gray value differences between overlapping pairs of sample pixels of the model image; computing a compact binary representation for each overlapping pair; computing an expanded binary representation for each compact binary representation; associating a corresponding pixel position with each expanded binary representation to provide a model element; and grouping all model elements together to provide a model, and at run-time, determining gray values at a plurality of selected pixel locations in a run-time image in accordance with pixel positions of the elements of the model; computing gray value differences between overlapping pairs of the selected pixels of the run-time image; computing a compact binary representation for each overlapping pair of the selected pixels of the run-time image; computing a run-time bit-expanded representation for each run-time compact binary representation; comparing each run-time bit-expanded representation with a corresponding model bit-expanded representation to provide a match count increment; and combining all match count increments to provide a total match value.