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As earlier described in the context of active-appearance models, a multi-resolution approach is used to speed up the whole process. Blurring or averaging followed by scaling allows for images of smaller size to be manipulated and complexity to be quadratically reduced. As the similarity measures are proportional to the images size, far better performance can be achieved by a transition from coarse to finer resolution. Pluim [37] identifies the effects that this approach will have on the measurement of similarity.