Digital watermarking system for halftone images based on particle swarm optimization

Halftone images are widely used in the printing of books, magazines, newspapers and computer printers. To protect the copyright, in this paper, the multi-purpose watermarking system is proposed for halftone images with the dithering technique and particle swarm optimization (PSO). The alternation of dispersed dot order dither cells generated by the PSO achieving good robustness and image quality produce patterns as embedding watermarks. For the purpose of copyright protection, the watermarks are embedded into the halftone image with visually transparency by the variety of dither arrays, robust to malicious attacks and counterfeits. And for the purpose of content authentication, the areas with malicious attacks could be detected and localized by fragile watermarks. In the system, the geometric invariance method, multi-rings Zernike transform (MRZT)containing co-centric rings sub-sampling and the clustering method, are adopted to estimate the angle and scaling factor accurately and achieve geometric invariance. The experimental results show that the watermarking algorithm provides the properties of robust and fragile simultaneously for halftone images.And the watermarked halftone image contains high image quality by the PSO algorithm.