Rational Dither Modulation has been proposed as an effective QIM watermarking algorithm which is robust against value-metric scaling. Invariance is obtained by quantizing a rational function of the host features instead of the features themselves. In this paper we propose a vector extension of the basic RDM scheme. Specifically, a sequence of feature ratios is quantized vectorially with the aid of a properly designed dirty trellis code. A fast sub-optimum embedding algorithm is proposed ensuring fast watermark insertion and good distortion properties. Preliminary results show that a significant advantage is obtained with respect to conventional RDM.
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