A Novel Encoding Method for Ideal Contrast Visual Secret Sharing Schemes with Reversing

In visual secret sharing (VSS) schemes, a secret image can be visually revealed by overlapping shares. But the contrast of reconstructed image is much lost. A useful way to improve this situation is to employ reversing, a non-cryptographic operation supported by most copy machines, to reverse black and white pixels. Firstly, this paper gives a survey for current VSS schemes with reversing. Secondly, an improved version for Cimato et alpsilas scheme with ideal contrast is shown. It cuts down the encoding runs from m to m-h+1 where m is the pixel expansion. Thirdly, this paper presents a novel scheme with reversing in which a really ideal contrast can be achieved within m-h+1 runs and no pixel expansion occurs. It encodes a secret image block by block (consisting of m pixels), which means that m pixels together join each encoding run. Fourthly, another non-expansible ideal contrast scheme with multi-pixel encoding is also presented, but it is based on non-perfect black VSS scheme.