Abstract Visual cryptography is an important data encoding method, where a secret image is encoded into n pieces of noise- like shares. As long as there are over k shares stacked out of n shares, the secret image can be directly decoded by the human naked eye; this cannot be done if less than k shares are available. This is called the (k, n)-threshold Visual Secret Sharing Scheme (VSS). Progressive Visual Cryptography (PVC) differs from the traditional VSS, in that the hidden image is gradually decoded by superimposing two or more shares. As more and more shares are stacked, the outline of the hidden image becomes clearer. In this study, we develop an image sharing method based on the theory of PVC, which utilizes meaningful non- expanded shares. Using four elementary matrices (C0-C3) as the building blocks, our dispatching matrices (M0- M3) are designed to be expandable so that the contrast in both the shares and the restored image can be adjusted based on user needs. In addition, the recovered pixels in the black region of the secret image are guaranteed to be black, which improves the display quality of the restored image. The image content can thus be displayed more clearly than that by previous methods.

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[16] Wang R. and Shyu S., Scalable Secret Image Sharing, Signal Processing: Image Communication, vol. 22, no.4, pp. 363-373, 2007. Young-Chang Hou received his B.S. degree in atmospheric physics from National Central University, Taiwan in 1972, his M.S. degree in computer applications from Asian Institute of Technology, Bangkok, Thailand in 1983, and his Ph.D. degree in computer science and information engineering from National Chiao-Tung University, Taiwan in 1990. From 1976 to 1987, he was a senior engineer with Air Navigation and Weather Services, Civil Aeronautical Administration, Taiwan, where his work focused on the automation of weather services. From 1987 to 2004, he was on the faculty at the Department of Information Management, National Central University. Currently he is a professor with the Department of Information Management, Tamkang University, Taiwan. His research interests include digital watermarking, information hiding, fuzzy logic, genetic algorithms, and visual cryptography. Zen-Yu Quan received his B.S. degree in Information management from Tatung University, Taiwan in 2007, his M.S. degree in information management from Tamkang University, Taiwan in 2009. He is currently a PhD student at the Department of Information Management, National Central University, Taiwan. His research interests cover secret sharing, digital watermark, and image retrieval. Chih-Fong Tsai received a PhD at School of Computing and Technology from the University of Sunderland, UK in 2005. He is now a professor at the Department of Information Management, National Central University, Taiwan. His current research focuses on data mining and machine learning. He has published more than 50 professional publications and received the Emerald Literati Network 2008 Awards for Excellence from Online Information Review ( A Review of Image Retrieval Methods for Digital Cultural Heritage Resources ) and the award for top 10 cited articles in 2008 from Expert Systems with Applications ( Using Neural Network Ensembles for Bankruptcy Prediction and Credit Scoring ).