In the related works of progressive visual secret sharing (PVSS), Wang et al. (2007) and Wang (2009) all have the following drawbacks: (1) The design of the dispatching matrices is not regular. (2) Shares are m-times larger than the original secret image. (3) Restored image's quality is poor. (4) Their schemes are not suitable to apply in grayscale and color secret images. In order to solve the above limitations, we propose a non-expanded PVSS approach, in which the recovery method is based on secret blocks, namely Block-based Progressive Visual Secret Sharing (BPVSS). We divide a secret image into n non-overlapped image blocks, and an additional portion of secret will be restored after superimposing one more transparency. When superimposing any t shares, there have t (2≤t≤n) secret blocks being restored. Compared with other related works, BPVSS has several advantages: (1) The concept of this model is concise, easy to implement, and the number of participants will not be restricted. (2) In the situation of noise-like shares, the contrast of the restored image is 50%, which means that the hidden message can be clearly recognized by the naked eyes. (3) When transparencies are shifted from noise-like into meaningful, the contrast of the stego-image and the restored image will be 25% which is still superior to other related studies. (4) Our scheme is more suitable for grayscale and color secret images than previous related studies. (5) The size of transparencies is the same as the size of secret image.
