Influence of Hardcopy Types on Extraction Quality of Holographic Watermark

Abstract

In this paper, firstly the high-frequency coefficient of Fourier hologram watermark was embedded into the middle-frequency of the host image subject to wavelet transform for twice. Then four common hardcopy types (inkjet, laser, offset printing and digital printing) were used to output watermarked images. Finally the watermark extraction quality of the hardcopy images was evaluated from paper surface quality, color space conversions of images and output resolution by using NC parameters. Experiment results showed that the watermark extraction qualities between digital image, presswork and printing are very different, and the rank is: inkjet printing>offset printing>digital printing and laser printing. The printed substrate paper with higher whiteness and gloss is beneficial to watermark extraction. The color space conversions of watermarked image in input and output process affected color distribution and image reproduction quality of original image, then the watermark extraction quality was influenced. The hardcopy resolution has little effect on watermark extraction quality on the premise of satisfying the image reproduction quality. Therefore, actual hardcopy type should be used instead of simulated attacks or desktop printing when studying the anti-print/scan robustness of watermarking algorithm. The substrate paper with high surface quality facilitate the improvement of watermark extraction quality. And the resolution is appropriate as long as the output reproduction quality is satisfied when verifying the anti-print/scan robustness of watermark scheme.