Visual cryptography encodes a secret binary image SI into n shares of random binary patterns. The secret image can be visually decoded by superimposing a qualified subset of shares, but no secret information can be obtained from the superposition of a forbidden subset. Such a scheme is mathematically secure, however, the binary patterns of the n shares have no visual meaning, raising the suspicion of data encryption. In order to achieve a higher level of security, halftone visual cryptography was proposed to encode a secret binary image into n halftone shares (images) carrying significant visual information. The method is further extended in this paper. Based on blue-noise
dithering principles, a global optimization method is proposed to
improve the overall visual quality of all n halftone shares.
Thus, the adversaries are less likely to suspect the presence of
hidden cryptographic information.