The present invention provides a method for the calculation of a pattern similarity metric that is locally normalized with respect to illumination intensity, and is invariant with respect to rigid body preserving gray scale variations, such as scale, rotation, translation, and non-linear intensity transformations. In one aspect, the invention provides a method for comparing a model image with a run-time image so as to provide a quantitative measure of image similarity. In another general aspect of the invention, a method is provided for searching a run-time image with a model image so as to provide at least one location of the model image within the run-time image.