Progressive sharing of an image

We propose a sharing method to progressively reveal a given important image in the recovery phase. In the encoding phase, the distributor utilizes the three frequency bands (low, middle, and high) of the given image to generate shadows according to three prespecified thresholds. In the recovery phase, the secret image cannot be revealed if the number of shadows a team collects is less than the lowest threshold. However, when the number of collected shadows reaches the prespecified low (or middle- or high) threshold, the team can reconstruct a low- (or middle- or high-) quality version of the secret image. In other words, the quality of the reconstructed image depends only on the number of shadows being received, rather than on which of the generated shadows are received. Each noise-like shadow is so small that it can be hidden in an ordinary image that is still several times smaller than the original image.