Informed coding for color hardcopy watermarking

Abstract

We investigate informed coding and multi-bit spread spectrum modulation framework for watermarking color images intended to be deployed as printed media. We propose improvements on the embedding and the detection watermarking techniques, aiming for improved color hardcopy watermarking using inkjet printers. The modulation approach allows to increase the watermarking robustness to achieve high payload with good transparency. At the encoding phase the impact of the color background on the detection metric is evaluated based on the estimated ink color correlation between the watermark and the document. This correlation is used to estimate and compare the robustness of sets of patterns which would be otherwise considered equivalent for modulating the same message. This approach allows a proper selection of a set of sequences to be embedded for improved robustness. The detection is further improved by using a proposed whitening filter and a color rejection algorithm. Examples using real life images illustrate the performance enhancement achievable by employing the proposed approaches in color hardcopy watermarking systems.