Halftone visual cryptography
Visual cryptography encodes a secret image SI into n shares of random patterns. If the shares are xeroxed onto transparencies, we can visually decode the secret image by superimposing a qualified subset of transparencies, but no secret information can be obtained from the superposition of a forbidden subset. Such a scheme is mathematically secure, however, it produces random patterns which have no visual meaning, raising the suspicion of data encryption. In this paper, to achieve a higher level of security, we propose halftone visual cryptography, where all shares are halftones of grey level images carrying significant visual information. The proposed methods utilize blue-noise dithering principles to construct halftone shares having visually pleasing attributes.