Modulation in the HVS Domain for Hardcopy Watermarking of Color Documents

This paper describes a novel color hardcopy watermarking embedding. The proposed color embedding provides high transparency by modulating the watermark into the value and saturation color components of the document. After the color document is printed and distributed, the message is decoded by scanning the document and applying the proposed post processing based on a Laplacian of Gaussian whitening filter. The decoding is performed in the frequency domain for speed and the optimal pattern detection threshold is determined by the analysis provided. An error correction code approach is proposed and the probabilities of false positives and false negatives for the embedding patterns are provided in the experiments for some printer and scanner devices. The examples illustrate the message coding using a combinatorial encoding and time division modulation to achieve with our error correction code approach a message payload of 168 bytes in a color image of about 2400 × 3200 pixels and estimated decoding probability per message of 1.3E-7 for inkjet printer and 2.6E-10 for laser printer. The proposed modulation provides a very high perceived transparency. The objective resulting quality is contrasted with the perceived quality of the embedding by employing 3 objective metrics: PSNR, SSIM on the RGB and on HVS color models. For the proposed modulation, the SSIM on the HVS is the metric with best correspondence with the perceived quality by the human visual system.