How to construct progressive visual cryptography schemes

A visual cryptography scheme (VCS) is an encryption method for images that does not need a computer to decode a secret image. In this paper we propose a simple, progressive VCS which is constructed from a traditional threshold VCS. Our progressive VCS has better decoded image quality and flexibility than the Fang-Lin scheme, a previous progressive VCS. We also present a block-wise progressive VCS which has jigsaw puzzle like decryption, and an XOR-based progressive VCS without pixel expansion. We give some experimental results which show that our scheme is superior to the Fang-Lin scheme with respect to the decoded image quality and the decoding speed controllability.