Visual Cryptography for Natural Images and Visual Voting

Visual cryptography is a type of secret sharing which encodes a secret image into several shadow images in such a way that the stacking of certain images printed on transparencies will reveal the secret. The decryption is done directly by the human visual system without any extra calculations. Most of previous researches essentially handle only binary images, as the underling encoding matrices are all Boolean matrices. For gray-level image, we need to halftone it into binary image before encoding. Although binary image can be used to simulate gray-level image, its visual quality is deteriorated, especially for fine images. The first part of this paper presents a method to provide much more gray-levels than previous schemes, given the same pixel expansion, and thus establishes the visual cryptography scheme suitable for natural images. The second part of this paper presents a visual voting scheme that need no counting process and guarantees anonymity.

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