Colored visual cryptography scheme based on additive color mixing

A (t,n)(t,n) visual cryptography scheme (VCS) encodes a secret image into n shadow images (shadows) distributed among n participants. When any qualified set of at least t participants stack their shadows, the secret image can be visually revealed. The first VCS proposed by Naor and Shamir was only designed for encrypting a black-and-white secret image; subsequently, some colored VCSs (CVCSs) for sharing colored images were proposed to enhance the applicability. In CVCSs, we usually use several color subpixels to represent a secret pixel where the number of subpixels is referred to as the pixel expansion. Generally, the CVCS requires a larger pixel expansion to produce more colors; however, this expansion will increase the shadow size. We therefore study the additive color mixing in a probabilistic way and propose a new CVCS whose pixel expansion is fixed and improves on the previous CVCSs, at the price of reducing the contrast quality to certain level.