7. The method of claim 1 wherein the machine-readable plural-bit auxiliary signal is only detectable either at the first observation angle or the second observation angle, but not at both angles.
7. The method of claim 1 wherein the machine-readable plural-bit auxiliary signal is only detectable either at the first observation angle or the second observation angle, but not at both angles. 8. A computer readable medium comprising instructions or circuitry to perform the method of claim 7 .