| Compared with traditional fake protection technology, digital watermarking technology has become more and more important in copyright protection and product fake protection. Digital watermarking technology is a new orientation of information security in the metaphase of 90'. It is the newest multimedia secrecy technology which uses a method of information processing to imbed watermark into digital multimedia data, make the apperceive system of human can not be aware of it and it only can be extracted through special detector. Currently, more research is applying digital watermarking technology to copyright protection, pirate tracking, covert communication, visiting control and copy control of multimedia production. Presently, print and scan process is widely used in image produce and issue, and it is a common mode to copy and promulgate image. With the rapid development of digital technology, it is very easy to transform image between digital format and print format. If watermark can be detected from the image which undergoes print and scan process, then digital watermarking technology can be applied in more widely field, such as identification document protection and bill fake protection, etc. so applying digital watermarking technology in printed matter has become a very important topic. With the increasing in printed matter fake protection demand of certificate, securities, art work and so on, the application of using watermarking technology into print fake protection field has much develop potential and social significance.The image which undergoes print and scan process looks similar as the original one, but actually the pixel value of the image has changed a lot, scanned images often have different degree of luminance and contrast grade variety, and also have some distortion brought by rotation, scaling and shearing, these image distortion will bring much difficulty to watermark detection. Experiment indicates that most of watermarking algorithm can not resist pixel distortion and geometrical distortion (like rotation, scaling ,shearing. etc), so looking for the changeless image character of print and scan process and designing the algorithm which can resist print and scan process has become a research difficulty of digital watermarking technology. This paper mainly aiming this difficult problem, bring forward effective digital watermarking algorithm.The paper is divided into six chapters, the detailed arrangement as following: In Chapter One, overview the demand background and research developments of digital watermarking technology, mainly introduce the research of present situation of digital watermarking technology which can resist print and scan, bring forward present problem and solving method. Based on application demand and advantage of applying digital watermarking technology into printed matters fake protection, the paper establish research purpose and content.In Chapter Two, correlative knowledge of digital image watermarking technology is introduced, bring forward basic demand for the design of image watermarking algorithm of after chapter.In Chapter Three, theoretical model of resisting print and scan digital watermarking is established, then analysis pixel distortion and geometrical distortion brought by print and scan process. Analyze in detail mainly aiming at pixel distortion problem, propose applying non-linear luminance transform and stretching contrast grade to adjust image pixel distortion. The parameter setting of these two adjusting method both use peak signal noise ratio as standard of measurement, even though using different printer and scanner, we can also get good adjusting image. Experiment can prove that using these two adjusting method can both improve watermark detection or extraction effectively. At last we design certificate fake protection system which uses digital watermarking technology, and propose special demand of using watermarking algorithm to actual application properly.In Chapter Four, based on the character that symbol of parts of image's DCT coefficient will not change, we propose self-adapting resist print and scan digital watermarking algorism based on partitioning DCT. This algorism imbed watermark adaptively, it can improve the capability of imbedded watermark and can also maintenance image quality. This algorism can imbed certificate number into certificate photograph to realize certificate anti-counterfeit. It has many features:①Watermark content imbedded is a significant sequence which based on certificate information;②Watermark imbedding algorithm has self adapting character, it can improve the capability of imbedded watermark and can also maintenance image quality.③It is a blind watermarking which fitted to actual application;④The computation complexity is not high, the rate of watermark extraction is no more than two seconds, so it can satisfy the demand of actual application;⑤There are two cipher key participated in the whole process, it can make the watermarking system more safety;⑥After set the threshold correctly, it will achieve a high veracity.In Chapter Five, resisting print and scan digital watermarking algorithm which based on spread coding is proposed. This algorithm adopt the strategy of spread coding, watermark image is produced due to the feather of image itself, it can improve the safety of watermark detection prominently. One remarkable virtue is that this algorithm can both resist to pixel distortion brought by print and scan process and to geometrical distortion including rotation, scaling, shearing brought by human factor. This algorism can be applied to certificate anti-counterfeit. The capacity of imbedding watermark is relatively large, there are cipher keys participated during imbedding and detecting, so it can improve the safety of watermarking system. Furthermore, the circulate rate of this algorithm is very fast, and hardly can not generate inaccurate detection.In Chapter Six, we summarize the whole paper, and show out some questions to be resolved in digital watermarking in the future.The main innovative work of this paper focuses on the following aspects.(1) In order to solve the pixel distortion problem brought by print and scan process, propose the algorithm that applying non-linear luminance transform and stretching contrast grade to adjust image pixel distortion. Simulation results show that both of these two adjusting method can improve the effect of watermark detection or extraction effectively.(2) Based on the character that symbol of parts of image's DCT coefficient will not change, we propose a new self-adapting resist print and scan blind watermarking algorism. Experiment result show that this algorithm can realize anti-counterfeit of soft-cover certificate using watermark technology.(3) In order to imbed large capacity of watermark, adopting the strategy of spread coding, resisting print and scan digital watermarking algorithm which based on spread coding is proposed. Watermark image is produced due to the feather of image itself, it can improve the safety of watermark detection prominently. Simulation results show that the algorithm can both resist to pixel distortion and geometrical distortion, so it can be applied in soft-cover certificate anti-counterfeit. |
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