Hybrid Watermarking Scheme for Halftone Images

As digital halftone images are widely used in the printing of books, magazines, newspapers and computer printers, the ease with which perfect copies can be made, may lead to large-scale unauthorized copying. Watermarking has been considered for many copy prevention and copyright protection applications. The hybrid watermarking system is proposed for halftone images that the watermarks embedded based on the spatial domain and frequency domain separately to enhance the robustness. For the purpose of copyright protection, the watermarks are embedded into the halftone image with visual transparency and robustness by the variety of dispersed dot order dither arrays generated by the particle swarm optimization (PSO) method achieving good robustness and image quality produce patterns for the spatial watermarks embedding system. After the binary pseudo-wavelet transform (BPWT), the second watermarks are embedded into the high-frequency components of the BPWT coefficients of the spatial domain based watermarked image that the frequency domain based watermarking is independent to the spatial domain based one. And the modified quality criterion system for halftone images is proposed by using the PSO based visual filter. The experimental results show that the hybrid watermarking algorithm is more robust for halftone images and the watermarked halftone image possesses high image quality by the PSO algorithm.