Image Content Restoration Fragile Watermarking Algorithm With Variable Capacity

This thesis firstly introduces the research background, significance and application of digital image watermarking technology, and simply analysis the basic knowledge, attack types and the evaluation index of self-recovery fragile watermarking algorithm. Secondly, we study the existing self-recovery fragile watermarking algorithms and point out that there are some contradictions between watermark capacity and quality of recovery, as well as contradiction between data synchronization attack and watermark-data waste. Moreover, the study for special image gets a lot less attention.To take watermark capacity and restoration quality into account, a variable-payload fragile watermarking scheme is proposed based on multiple description coding. Image block is divided into smooth, edge and texture blocks. For each block, the DCT coefficient is encoded to generate two independent and complementary description codes with different length based on adaptive classification of image blocks. Each description code can approximately restore the content of image block and two description codes can achieve the higher quality of recovered block. In the watermark embedding stage, two description codes are randomly embedded in other image blocks based on secret key, which not only improves the tamper detection performance under different attacks, but also reduce the possibility that the content and recovery watermark of an image block has been tampered simultaneously. The tampered image block can be approximatively restored as long as there is a valid description code. If there are two valid description codes of the tampered blocks, the quality of the recovered block can be further improved. This relieves the conflict of tampering coincidence and watermark-data waste. Results demonstrate that the proposed method effectively reduces the watermark capacity, and improves the tamper detection performance and the recovery quality under different attacks.In view of special purpose image, such as the electronic invoice, a variable capacity restore fragile watermarking algorithms is proposed for the protection of electronic invoice saved as GIF format. To take recovery quality and code-length into account, the image blocks can be divided into two categories according to the content importance:important block and unimportant block. For the important block, authentication information are generated and embedded in the itself. At the same time, the recovery watermark with variable length is obtained and hidden in the unimportant image block based on the secret key. For unimportant blocks only authentication watermark is generated and embedded in itself. In the tamper detection stage, the validity of each block is firstly initialized according to the consistency between the generated authentication watermark including type-code and extracted one. The authenticity of each important block is determined by comparing the consistency of the watermark recovery and combining with block4-neighborhood characteristics. For important image block which has been considered as tampering, the extracted recovery watermark is used to restore it. Experimental results show that the proposed algorithm can effectively resist the various attacks.Finally, we design the algorithm simulation system, in order to simulate and validation the proposed watermarking algorithm with self-recovery capability.