Expansion-free grayscale image sharing using meaningful share images

Since Naor and Shamir introduced visual cryptography in 1994, numerous image sharing techniques have been proposed to protect the security of binary, gray, and color images. However, only few of them focus on grayscale image sharing. Because the size of the share images are usually very large, an image sharing method that can create share images with the same size as that of the secret image (expansion free) is more attractive. This study aims to propose a grayscale image sharing method to achieve the purposes of free expansion and meaningful image sharing simultaneously. Experimental results show that two meaningful share images can be effectively created by the proposed share image creation process. Moreover, the original grayscale secret image can be correctly decoded by the proposed secret image retrieval process.