1. A method for determining whether a perceived target is acceptably close to a model target, said method comprising the steps of: segmenting the perceived target using a relaxation based procedure; extracting structural features from the perceived target by producing a compact one-dimensional description of the perceived target's boundary; and classifying said structural features by using string-to-string matching, wherein one of two symbolic strings is a representation of the compact one-dimensional description of the boundary of the perceived target, and the other of said two symbolic strings is a prestored representation of the model target, and the string-to-string matching entails measuring the distance between the two strings based upon deletion, insertion, and substitution of symbols from one string to the other; so that: performing the string-to-string matching measures how closely local structural features of the perceived target resemble local structural features of the model target.