The GC feature is computed by GC ??? ( p ) = 1 N ??? ??? i = l N ??? ??? i max ??? ( p ) ??? i max ??? ( p ) = max R min ??? n ??? R max ??? { 1 n - R min + I ??? ??? j = R min n ??? cos ??? ??? ?? ij ??? ( p ) } ( 25 ) [0210] where p is an operating point (OP), and N is the number of the symmetric directions used for computing the response (FIG. 21a).