Visual cryptography for print and scan applications

Visual cryptography is not much in use in spite of possessing several advantages. One of the reasons for this is the difficulty of use in practice. The shares of visual cryptography are printed on transparencies which needed to be superimposed. However, it is not very easy to do precise superposition due to the fine resolution as well as printing noise. Furthermore, many visual cryptography applications need to print shares on paper in which scanning of the share is necessary. The print and scan process can introduce noise as well which can make the alignment difficult. We consider the problem of precise alignment of printed and scanned visual cryptography shares. Due to vulnerabilities in the spatial domain, we have developed a frequency domain alignment scheme. We employ the Walsh transform to embed marks in both of the shares so as to find the alignment position of these shares. Our experimental results show that our technique can be useful in print and scan applications.