Wavelet Domain Adaptive Image Watermarking

With the rapid development of multimedia technology, all kinds of digital information such as images, video and audio can be easily created by digital devices. Meanwhile, the multimedia information is widespread distributed on the Internet. Because of easy reproduction, retransmission and even manipulation, it brings the problem of copyright protection and safe transmission of digital information. How to protect the copyright and solve the problem of safe transmission has become an important issue to researchers.Conventional cryptography can be used for access control, but the drawback of this data protection strategy is that once such a data is decrypted by a pirate, there is no way to protect the data and track the illegal distribution. Digital signature can provide integrity authentication, however, it has to embed large amount of information to the original data. Digital watermarking as an important branch of information hiding modifies the conventional cryptography. Not only can it provide further protection for decrypted information, but also can embed large amount of information. Therefore, digital watermarking has gained more and more attention and found great use in the fields of copyright protection, integrity authentication, access control, etc.This paper mainly focuses on the research of the digital image watermarking based on DWT (discrete wavelet transform). It first introduces the background of the digital watermarking and its current research and applications. Then it gives an overview of the watermarking system, including the definition, basic framework, characteristics, classification and common attacks. After that a survey of classic digital watermarking algorithms is illustrated in the paper. Recent watermarking algorithms can be broadly classified into three categories:spatial domain algorithms transform domain algorithms and compressed domain algorithms. The classic algorithms of the three categories are discussed according to current research. Because self-adaptiveness is very important to digital watermarking, several kinds of adaptive algorithm are surveyed as well. Then we discusses the theory basis of digital watermarking based on DWT-the wavelet theory.An adaptive image watermarking algorithm in wavelet domain is proposed in the paper. The proposed method can achieve better trade-off between invisibility and robustness. A perceptual model based on the computation of NVF (noise visibility function) is exploited in the proposed watermarking scheme. The characteristics of the original image are shown by using the NVF, and the embedding depth is decided by the model. In the embedding process, the original image is first transformed in the wavelet domain and the embedding factor is decided through computing the NVF of the mid-frequency; then the watermark is embedded in the selected subband with different embedding factors; finally, the watermarked image is obtained by reverse wavelet transform. Experimental results show that the proposed method is adapted to various gray images with different characteristics and robust to wide variety of image processing operations.The proposed algorithm can gain a good trade-off between invisibility and robustness. However, it needs to be improved in some aspects. For example, it is less robust to geometric attacks such as rotating and cropping; the arrange of original image can be expanded to color images, etc. In the future work, we will focus on these aspects.