Research On The Robust Watermarking For Color Image Oriented To Printing Fidelity

With the technology development of internet and hard copy,the process of graphic communication and reproduction is becoming easier and more convenient.Issues of illegally modifying or reproducing the original works with the digital image processing technology and high-quality input/output equipment are increasing,which means the need for effective way of copyright protection,forensics and anti-counterfeiting that aims at printing and digital images,is urgent.Digital watermarks as one of the effective methods for copyright protection,forensics and anti-counterfeiting for digital media,has been researched for years and lots of results has been achieved,comparatively,the copyright protection research for printed copies has just started.Aiming at the copyright protection of colored copies of printing fidelity,the paper studies the robust watermarking method based on the principles and characteristics of the printing process.The research investigates the methods of robust watermarking in three areas,the process based on the ICC color management technology,Non-ICC color management technology(such as HiFi printing technology)and spot color printing process.For the ICC color management printing process,a watermarking method based on the ICC color space transformation is present in the paper.The watermark information is embedded firstly in the Yellow-Blue color channel based on CIEL*a*b*color space which is device independent and then a template signal for watermark synchronization is also embedded in the Red-Green color channel in order to resist geometric distortion during print-scanning process and to avoid image degradation because of multiple embedding in Yellow-Blue color channel.The watermarked images is then screened with AM screening process and printed with lithography.De-screening is used for the printed-scanning images before detecting the watermark.According to the results,the algorithm that this paper presented preserve watermark's transparency well and it is robust to printing process,meanwhile,it is also robust to conventional image processing and geometric transformation.This method can be used synchronously in both digital and hardcopy color images for copyright protection.For the printing process that doesn't use ICC color management,a robust watermarking algorithm based on CMYK color mode is present in the paper,comparatively,current methods usually cause distortions for both watermark and color appearance during the transformation of RGB space to CMTK space.An informative watermark and a structure template signal were embedded in DCT domain of the Yellow information channel and DFT domain of the black information channel respectively.Another spatial visual mask is proposed to reshape the embedded energy after it is inverted to the spatial domain,which significantly improves the color fidelity and the image quality.In addition,a dot gain compensation model is put forward for color correction.A series of proof tests have been carried out on an offset press,which shows that the method can be applied to lithography for CMYK color mode fidelity printing.Based on the color space transforming principle and in view of Integer wavelet transform(IWT)and Singular value decomposition(SVD),a robust watermarking algorithm for spot color printing is presented herein,especially for the spot color printing which has limited color channels and low water mark capacity.In the first place,the original image is separated to spot-channel image,which is transformed with IWT,and the watermark is then embedded in singular values of the 1-level IWT decomposed sub-bands.To improve the accuracy in case of high false alarm rate,Arnold scrambling and encryption is adopted and a digital signature is embed in the watermarked images,so extracting digital signature is required prior to the extraction of watermark information.To improve the robustness,a spot color printing-scanning dot transfer curve based on the spot color scale is presented to obtain the spot color tone for scanning image correction.A series of tests were conducted with offset printing and the results showed that the method is effective in visual imperceptibility and anti-lithography process.