Contrast-Enhanced Color Visual Cryptography for (k, n) Threshold Schemes

In traditional visual cryptography schemes (VCSs), pixel expansion remains to be an unsolved challenge. To alleviate the impact of pixel expansion, several colored-black-and-white VCSs, called CBW-VCSs, were proposed in recent years. Although these methods could ease the effect of pixel expansion, the reconstructed image obtained by these methods may also suffer from low contrasts. To address this issue, we propose a contrast-enhanced (k, n) CBW-VCS based on random grids, named (k,n) RG-CBW-VCS, in this article. By applying color random grids, a binary secret image is encrypted into n color shares that have no pixel expansion. When any k₁ (k₁ ≥ k) color shares are collected together, the stacked results of them can be identified as the secret image; whereas the superposition of any k₂ (k₂ < k) color shares shows nothing. Through theoretical analysis and experimental results, we justify the effectiveness of the proposed (k, n) RG-CBW-VCS. Compared with related methods in feature, contrast, and pixel expansion, the results indicate that the proposed method generally achieves better performance.