Image watermark that is resistant to geometric distortion is
remained to be an unsolved problem. Difficulty of the probelm comes
from the situation that the watermark should be extracted without any
information of the original image. In this paper, we review this problem
and propose a new watermarking scheme based on invariant pattern
recognition theory. We propose an invariant watermark using the Radon
transform and higher order spectra. A bispectrum feature vector of the
image is used as the watermark. Our approach differs from the previous
methods in that we embed watermark into the phase of the higher order
spectra. Also, our Radon embedding grid outperforms the Fourier-Mellin
based methods. We devised a new embedding method which allows detection
of the watermark when there is no exact inverse function during
embedding. As we use the Radon transform, our method can be used for
medical images. We show the invariance of the designed watermark with
mathematical proofs. Experimental results confirm that this scheme is
resistant to geometric distortions.