Two of the main areas of research in visual cryptography have been on improving the visual quality of the recovered image and in reducing the pixel expansion of the shares. This paper addresses both of these visual cryptography issues. First, a method to enhance the perceived visual quality of the recovered image using various image filtering techniques is presented. In particular, these image filtering techniques are used to enhance the local and global contrasts of a grayscale image. Second, a multi-pixel block size invariant visual cryptography scheme that maintains the relative density of local neighboring pixels is proposed. This method encrypts blocks of pixels based on whether the total number of black pixels within the respective blocks is above or below a certain threshold. In conjunction, these approaches effectively improve on the perceived visual quality of a recovered visual cryptography image.
Chow, Y., Susilo, W. & Wong, D. (2012). Enhancing the perceived visual quality of a size invariant visual cryptography scheme. Lecture Notes in Computer Science, 7618 (2012), 10-21.