Improved techniques for watermarking halftone images

Watermarking schemes that have been developed for continuous tone images cannot be directly applied to halftone images. Many of these watermarking methods require characteristics implicit in continuous tone images but absent from halftones. With this in mind, it seems reasonable to have a robust watermarking technique specific to halftones, equipped to work in the binary image domain. The paper reviews existing techniques and suggests improvements to increase their performance and overcome their limitations. The first set of post-halftone watermarking methods work on existing halftones. Data hiding cell parity (DHCP) works in the parity domain instead of on individual pixels. Data hiding mask toggling (DHMT) works by encoding two bits in the 2×2 neighborhood of a pseudorandom location. Dispersed pseudorandom generator (DPRG), on the other hand, is a preprocessing method that takes place before halftoning. DPRG achieves better visual results. Using the modified peak signal-to-noise ratio (MPSNR) metric, the proposed techniques outperform existing methods by up to 5-15%, depending on the image type and method considered.