16. The method as in claim 15 which further comprises the steps of: (a) determining the center of mass of the reference feature vectors of said selected class which are contained in said region; (b) determining the reference feature vector of said selected class which is contained in said region and which is closest to said center of mass; (c) using said reference feature vector which is closest to said center of mass as a center of region; (d) forming the substantially largest possible region around said center of region; (e) repeating steps (a), (b), (c) and (d) until either a predefined number of iterations is performed or said center of region is equal to said center of mass; and (f) selecting as said region, the region formed by step (d) which is either the last such region formed by step (d) or the largest of such regions formed by step (d).
19. The method of claim 16 wherein said center of mass is determined using said reference feature vectors of said selected class which are contained in said region and which are not contained in a previously formed region.