Threshold visual secret sharing by random grids with improved contrast

• We improve the contrast of Chen and Tsao's (k, n) threshold scheme published in The Journal of Systems and Software 84 (2011) 1197–1208.
• For (k, n) access structures satisfying k > n/2, the proposed scheme works the same way as Chen and Tsao's scheme.
• For (k, n) access structures satisfying k ≤ n/2, the contrast of the proposed scheme is strictly larger than that of Chen and Tsao's scheme.
• Computer simulations for (2, 4) access structure show that our scheme outperforms Chen and Tsao's scheme significantly in visual quality.

A (k, n) visual cryptographic scheme (VCS) is a secret sharing method, which encodes a secret image S into n share images in such a way that the stacking of any more than or equal to k share images will reveal S, while any less than k share images provide no information about S. Kafri and Keren (1987) firstly implements (2,2)-VCS by random grids (RG-based VCS). Compared to conventional solutions of VCS, RG-based VCSs need neither extra pixel expansion nor complex codebook design. However, for a long period, RG-based VCSs are confined to (2,2) access structure. Until recently, Chen and Tsao (2011) proposed the first (k, n) RG-based VCS. In this paper, we improve the contrast of Chen and Tsao (2011)'s threshold scheme. The experimental results show that the proposed scheme outperforms Chen and Tsao (2011)'s scheme significantly in visual quality.