Research On Digital Watermarking Algorithm Based On Fractional Fourier Transform

With the popularization of computer network and multimedia technology, the digitized communication provides a convenient way for creating, spreading and releasing digital products. Media information can be illegally copied, misappropriated and manipulated in an open network environment, which makes security protection and integrity verification of digital products become an urgent problem. The inapplicability of traditional information security technology for copyright problem promotes the development of digital watermarking. Digital watermarking technology embeds watermark signal into multimedia products to provide copyright proof for the ownership of digital media. This thesis mainly focuses on the study of fractional Fourier transform and discusses the practical application of the FRFT in digital watermarking. In this dissertation, a still image is taken as the research object. Combining characteristics of the quantization, norm and texture, two robust watermarking algorithms based on FRFT with better performance are proposed. The main work and innovations of this thesis are summarized as follows:(1) A novel robust image watermarking algorithm based on fractional Fourier transform and spread transform dither modulation is presented. First, lifting wavelet transform is performed on the original image. The lowpass subband is segmented into non-overlapping blocks, and each block is transformed by the fractional Fourier transform. Second, the low-frequency coefficients of the amplitude information are chosen to construct host vectors according to frequency. In the process of embedding watermark, spread transform is performed on host vectors to obtain the projected host vectors, and then a binary watermark bit is embedded using dither modulation. Finally, we use the minimum distance decoder to decode the watermark. The experimental results indicate that the proposed algorithm can obtain better transparency and robustness. Compared with previous similar schemes, the presented method achieves higher practicality and reliability.(2) Aiming at improving robustness and embedding capacity of existing algorithms, an adaptive robust image watermarking algorithm using vector norm based on fractional Fourier transform is presented. First, the original image is segmented into non-overlapping blocks. After FRFT is applied to the image blocks, the watermark is embedded in the vector norm of the segmented amplitude components through the relationship embedding strategy of the norm value. Moreover, an adaptive method is designed to adjust the embedding strength according to texture features which are calculated by gray level co-occurrence matrix. This method manages to relieve the “blocking effect” phenomenon in smooth regions of the host image. Simulation shows outstanding robustness against a wide class of common attacks and the method outperforms the quantization-based watermarking in FRFT domain in terms of robustness.