r transform means, each Fourier transformed reference pattern is defined by a formula of (X.sub.r +Y.sub.r j) where X.sub.r is a corresponding real part and Y.sub.r is a corresponding imaginary part, each of the valuesX.sub.r and Y.sub.r being a matrix consisting of N.times.N values, and the single Fourier transformed target pattern is defined by a formula of (X.sub.t +Y.sub.t j) where X.sub.t is