Research On Semi-Fragile Watermarking Technology For 2D CAD Engineering Graphics

Digital watermarking technology for 2D Engineering graphics involving cryptography, image processing, information security and many other subjects is an important filed of digital watermarking research. This thesis focuses on the fragile watermarking techniques for 2D CAD Engineering graphics. Firstly, the thesis expounds the background, feature, and current research status of the two-dimensional engineering graphics watermarking. Bases on these, the shortcomings in the existing watermarking algorithms are also analyzed. Secondly, the basic theory used in this thesis, including Chaos Theory, MD5, polar coordinates transformation, odd-even quantization and their respective features are introduced. Finally, two semi-fragile watermarking schemes for 2D CAD Engineering graphics are proposed, and their performance are discussed in detail in this thesis.(1) A semi-fragile watermarking algorithm for authenticating 2D CAD engineering graphics based on log-polar coordinates mapping is proposed. Firstly, the coordinates of vertices of entities are mapped to the log-polar coordinate system to obtain the invariance of translation, scaling and rotation. Then the watermark is embedded in the mantissa of the real-valued log-polar coordinates via bit substitution. Theoretical analysis and experimental results show that the proposed algorithm is not only robust against incidental operations such as rotation, translation and scaling, but also can detect and locate malicious attacks.(2) A semi-fragile watermarking algorithm for 2D CAD engineering graphics based on improved odd-even quantization is proposed. For the high false-positive rate in traditional odd-even quantization method, an improved quantization method is put forward, which is suitable for fragile watermarking. In this scheme, the entity set is firstly acquired from the 2D CAD engineering graphic, then they are arranged in groups, finally the watermark is embedded in each group with the method of odd-even quantization. Experimental results and analysis show that the proposed algorithm is robust against some incidental operations such as rotation, translation, scaling, but it is sensitive to malicious attacks and can detect and locate the attacks correctly.The watermarking approaches proposed in this paper can be used to integrity content authentication of 2D CAD engineering graphics, and will play an effective role in the promotion and application of the watermark technology on two-dimensional engineering drawing.