Visual Cryptography Using Computation-Free Bit-Plane Reconstruction

Abstract

Visual cryptography (VC) using bit-plane decomposition improves the quality of the reconstructed image. The disadvantage of this scheme is that the decoder needs computation in order to reconstruct the secret image from its bit-planes. To solve this problem, we propose a no-computation bit-plane decomposition visual cryptography (NC-BPDVC). In NC-BPDVC, we convert the grayscale secret image into a multitone image by multilevel halftoning. Then, by exploring the difference between a digital pixel and a printed dot, we design different dot patterns to render a digital pixel. By doing so, we abandon the usual assumption that DPI (dots per inch) equals PPI (pixels per inch) during printing. By adopting the more realistic assumption that DPI can be larger than PPI as is supported by most printers, we use different patterns to render different tone levels. These patterns are carefully designed so that no computation is needed when one needs to reconstruct the multitone image from its bit-planes. Our algorithm is tested on a batch of twenty standard grayscale images. The experimental results confirm the correctness and advantages of the proposed scheme. Compared with the ordinary bit-plane decomposition VC, NC-BPDVC does not need computation. The security of the proposed algorithm is also analyzed and verified.