Research On Reversible Image Watermarking Schemes

Digital watermarking is some labels or signatures which are embedded into digital multimedia host data by means of signal processing. The labels or signatures usually are not noticeable. It plays a very important role in the applications of copyright protection, content authentication and fake defending.The cover images will experience some imperceptible, irreversible or permanent distortions due to digital watermarking and the watermarked image can not be inverted back. But for some critical applications such as the fields of the law enforcement, medical and military image systems, it is crucial to restore the original image without any distortions after the watermark is extracted. The watermarking techniques satisfying these requirements are referred to as reversible watermarking.In this dissertation, we firstly summarize a systematical introduction of digital watermarking; then mainly focus on the reversible watermarking algorithm. The valuable research fruits can be summarized as follows:(1) A reversible watermarking scheme based on pairwise difference adjustment (PDA) and sorting technique is presented. In a grayscale image, every two adjacent pixels are grouped into a pair. The location map which records these changeable positions can be efficiently compressed by using PDA technique, and the capacity can be increased. At the same time, we use sorting technique to improve the quality of watermarked image as much as possible.(2) A reversible watermarking based on four-pixels PDA is proposed. Every four adjacent pixels are grouped into a tetrad. The scheme takes full advantage of PDA technique by three times in every tetrad to enhance the embedding ratio. Our method can achieve embedding rate of up to 3/4 bpp at once embedding.(3) A new watermarking scheme is presented aiming at JPEG images. In practice, most of images are saved or transported in JPEG format. Our method hides data in the quantified DCT coefficients of JPEG images, which achieve a high capacity. We can also change the capacity discretionarily by setting a threshold to obtain a nice image. Compared to other methods, our method has the advantage on holding the file sizes.