Study Of Imperceptibility Evaluation Of Watermark In Watermarked Digital Image

A number of robustness algorithms were proposed with the decades of development of digital watermarking study. Because of the differences of the evaluation criteria which were used, the advantages and disadvantages of watermarking algorithms were difficult to assess. So a reasonable performance evaluation criterion has become a powerful guarantee for promoting the research and application of digital watermarking technology. In this dissertation, the following research work has been accomplished, which is focus on the imperceptibility evaluation of watermark in watermarked image.1. The basis theory of digital watermarking was discussed. The importance of human visual system characteristics in digital watermarking technology research and development was analyzed, as well as the currently used fidelity evaluation methods of watermarking system, and their merits and demerits.2. The Watson's DCT-domain just noticeable difference model was analyzed. Some structural problems of the Watson's model were pointed out and amended by analyzing, and the correctness of the analysis and amendment was proved by experiments. The visual masking threshold was calculated by the algorithm which based the original visual model and the modified one, respectively. The different quality weights have been set up for diverse DCT coefficients in the images. Then the evaluation method was proposed based on the visual model. And the traditional Peak Signal to Noise Ratio assessment model was optimized.3. The discrete cosine transform was used as a signal analysis tool. Combing the psychophysics study, the frequency, texture and the luminance masking on human vision in image are analyzed. A noise-vision function of DCT coefficient was provided. As this basis, an imperceptibility evaluation method of watermark in watermarked image was proposed based on DCT coefficients. Experimental results show, the quality value calculated by this algorithm has high relevance and consistency with human visual sensation.4. The discrete wavelet transform was used as a signal analysis tool. The luminance, multi-scale texture and frequency masking strength were analyzed. The masking was combined with color masking, a JNCD (Just Noticeable Color Difference) was provided. As this basis, an imperceptibility evaluation method of watermark in watermarked color image was proposed based on DWT coefficients. Experimental results show, the quality value calculated by this algorithm has high relevance and consistency with human visual sensation. Especially the color difference determinant by this method is more consistent with the human eyes sensation.5. An imperceptibility evaluation of watermark in watermarked color image which based on brightness/chromaticity space was proposed. This algorithm is based on the brightness/chromaticity space which consistence with the human vision to color, the visual masking to color image was analyzed. It has weakened the influence by the asymmetric of the CIExy color space. Combing the luminance masking analytics method base on psychophysics study with the texture analytic method based on gray difference statistics, the noise masking strength of image was calculated. Then the brightness and chromaticity components were respectively transformed to their DWT, and the DWT coefficients were weighted by the above masking value. The experimental results showed that this evaluation method have high relevance and consistency with the subjective evaluation.