Visual cryptography scheme for gray-scale images with optimal contrast
具有优化对照度的灰度图像的可视分存方案

Visual cryptography scheme (VCS) is an encryption technique that utilizes human visual system in the recovering of the secret image and it does not require any complex calculation. Contrast has been a major issue of VCS schemes. Most copy machines have reversing function, in which can change a black image into a white one and vice versa. By means of the reversing operation, perfect black binary VCS with reversing can obtain ideal contrast. In this paper we conclude that we do not use existing VCS with reversing to construct an ideal contrast VCS for gray-scale images, then we show how to construct an ideal gray-scale VCS with reversing. In our scheme each participant is required to store a certain number of shares,and the number is same as that of binary VCS with reversing. Furthermore, the scheme guarantees that reconstructed image is correct since we perform certain runs operations.