A digital signal is imperceptibly embedded into an input source signal, such as an image or video signal, to produce an encoded (sometimes termed "watermarked") signal. The principle of quasi-rotational symmetry is employed to facilitate detection of the embedded signal notwithstanding rotation...http://www.google.com/patents/US5748783?utm_source=gb-gplus-sharePatent US5748783 - Method and apparatus for robust information coding
