<title>Digital watermarks using stochastic screens</title>

Joint Screening Halftoning and Visual Cryptography for Image Protection

“…On the other hand, if the size of an image database is large, then more random screens are required in order to make sure that any two images are generated from different screen combinations. As a result, how to generate sufficient number of screens is problematic for a large image database in [7]. In order to deal with these problems, we propose a new method, which relies only on a pair of conjugate screens.…”

“…In [7], Knox proposed to generate halftone images by means of combining one or more stochastic (random) screens. However, we find that combination of random screens results in poor quality of halftone images because random screening cannot disperse block and white dots uniformly, as shown in Fig.…”

“…In this work, we investigate a joint screening halftoning and visual cryptography scheme for image copyright protection. The major differences distinguishing the state of the art technology presented by Knox [7] from ours include: (1) halftone images with better quality can be obtained by two conjugate basic screens; (2) the size of a halftone image database is not limited.…”

“…Let B 1 and B 2 , which are independent uniform random variables within the interval [0, 1], denote the average dark degree in the dark area of the first and second share images, respectively. We will now analyze and compare the achievable average dark degree between random screening [7] and our screening halftoning-based method. In the random screening process, by considering n random screens the average dark degree can be derived as:…”

“…In order that the hidden information can be extracted by overlapping any two images, Knox [7] proposed a novel halftone image watermarking scheme based on stochastic screen patterns. In this method, a stochastic screen block is first selected and one or more than one stochastic screen blocks that are related to the first one are derived.…”