Image-adaptive watermarking based on perceptually shaping watermark blockwise

摘要

In a general additive watermarking model, a watermark signal is perceptually shaped and scaled with a global gain factor before embedding. This paper presents a new image-adaptive watermarking scheme based on perceptually shaping watermark blockwise. Instead of the global gain factor, a localized one is used for each block. And Watson's DCT-based visual model is adopted to measure the distortion of each block introduced by watermark, rather than the whole image. With the given distortion constraint, the maximum output value of linear correlation detector is derived in one block, which demonstrates the reachable maximum robustness in a sense. Meanwhile, an extended perceptually shaped watermarking (EX-PSW) is acquired through making detection value approach that upper limit. It is proved mathematically that EX-PSW outputs higher detection value than perceptually shaped watermarking (PSW) with the same distortion constraint. We also discuss the adjustment strategies of parameters in EX-PSW, which are helpful for improving the local image quality. Experimental results show our scheme provides very good results both in terms of image transparency and robustness.