User-friendly threshold visual cryptography with complementary cover images

User-friendly threshold visual cryptography schemes (denoted (k,n)-FVCS) generate meaningful shares to cope with the management problem of conventional threshold visual cryptography. However, several issues remain in this research field, including: the pixel expansion problem, the residual traces problem, inter-images interference on meaningful shares, the inability to adjust visual quality, the lack of systematic encryption methods, and others. To solve these problems, this paper proposes a novel (k,n)-FVCS for sharing binary secret images in non-computer-aided decryption environments. Based on the probabilistic (k,n)-VCS, we propose a systematic method for embedding complementary cover images in shares for constructing the (k,n)-FVCSs. The proposed approach can address the above-mentioned issues of (k,n)-FVCSs for all k and n. In addition, the experimental results indicate that the proposed (k,n)-FVCS significantly outperforms the previous methods in terms of the visual quality of the recovered images and of the meaningful shares.