Progressive Visual Cryptography with Friendly and Size Invariant Shares

Visual cryptography is an important data encoding method, where a secret image is encoded into n pieces of noiselike 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 nonexpanded 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.