A subjective study of visibility thresholds for wavelet domain watermarking

Ensuring watermark invisibility in digital images is a challenging task. Most watermarking techniques empirically adjust a strength parameter in order to reach the best trade-off between invisibility and robustness. A target PSNR value is typically set in order to reach a defined quality level. Some watermarking techniques exploit local activity to increase the watermark strength in some specific images areas (edges, textures). In this work we study the visibility thresholds for wavelet domain multiplicative embedding using a watermarking oriented subjective experiment protocol. Thirty four observers were enrolled for a subjective experiment, and had to adjust the watermark strength in order to best define the visibility threshold for watermarking distortions. Three watermark embedding equations were tested in various wavelet sub-bands, and the optimal equation maximizing robustness was derived for every sub-band.