The Model
A possible scenario for the proposed model involves a "dealer" and "participants" to the scheme. The dealer chooses a secret message represented as a binary black-and-white image and applies a (2,2) visual-cryptography scheme on the secret message, obtaining the two corresponding shares. Every share is individually embedded into an 8-bit grayscale image (traditionally called a "vessel") using the modified BPCS scheme. Finally, the dealer electronically sends the images with embedded data to the participants. Participants process the received image (also based on the modified BPCS scheme), obtain the embedded share as a binary image, and print the binary image on a transparency. As soon as the participants come together, they can visually reconstruct the secret message by carefully superimposing the two transparencies.
This model follows the block diagram in Figure 4. Though separated in practice, for simplicity, Figure 4 illustrates the embed/extract process together: only the steganographic method for an 8-bit grayscale image is treated. Figure 5 presents a general view considering the modality the data is embedded into the vessels.
