Aspect Ratio Invariant Visual Cryptography Scheme with Optional Size Expansion

Traditional visual cryptography scheme has a shortcoming of size expansion of the revealed image. Usually, the expansion ratio m is not a square integer, and results in the variant aspect ratio of the revealed image. The large size expansion is not convenient for storage and transmission. In this paper, an improved aspect ratio invariant visual cryptography scheme with optional size expansion is proposed. The details of the secret image have a priority to be preserved in the revealed image. Theoretically, the size expansion can be any value larger than 0. Compared with the literature schemes, our scheme can preserve the details well, especially for the thin lines in the secret image.