Visual Cryptography, Weighted Sharing, Damage Repairing, and High Quality Hiding of Images

• Main Authors: Lin, Sian-Jheng, 林憲正
• Other Authors: Lin, Ja-Chen
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/32877555637020716032

博士 === 國立交通大學 === 資訊科學與工程研究所 === 99 === In the dissertation, several technologies to protect digital images are proposed. To diffuse the content of a secret image, Visual Cryptography and Sharing are both studied in the dissertation. First, a Flip Visual Cryptography system is proposed. Two secret images are encoded as two transparencies. Stacking the two transparencies can reveal one secret, and the second secret is revealed by stacking the second transparency with the flipped version of the first transparency. The proposed scheme is proved to have perfect security and conditionally-optimal contrast. In the second topic of the dissertation, a weighted secret image sharing method is proposed. Each shadow has a weight which indicates the shadow’s relative importance. The secret image can be decoded as long as the total sum of received weights reaches a specified threshold. The weighted sharing method is based on polynomial division, and the traditional polynomial-style secret image sharing is a sub-case of ours. Moreover, by observing the characteristics of GF(2k), a fast sharing algorithm is also proposed. In the third topic of the dissertation, in order to have confidential transmission, an image-hiding method based on weighted-sum function is proposed. Compared with previous works, the proposed method has following advantages: i) wide range of embedding rate (such as 0.5 to 4.0 bits per pixel); ii) competitive PSNR values over the whole wide range; iii) once an embedding rate is given, our look-up table can predict the PSNR value, even before the actual embedding. In the fourth topic of the dissertation, in order to protect the accuracy of an image, a semi-fragile watermarking method with recovery ability is proposed. The recovery data are shared among many shadows; then each shadow is embedded in the DCT domain of an image block. When the watermarked image is tampered with in some area, the non-damaged shadows are extracted from image blocks; then the recovery data are decoded from those non-damaged shadows. The recovery data are then used to repair the damaged area. Experiments show that the proposed method can resist some content-preserving operations within certain degrees, such as JPEG compression, Gaussian noise, and brightness adjustment.