Usually, the reconstructed image will be darker than the background of the image itself. In this paper we consider visual cryptography schemes satisfying the model introduced by Tzeng and Hu (Designs, Codes and Cryptography, Vol. 27, No. 3, pp. 207-227, 2002). In such a model the recovered secret image can be darker or lighter than the background.
We prove a lower bound on the pixel expansion of the scheme and, for (2,n)-threshold visual cryptography schemes, we provide schemes achieving the bound. Our schemes improve on the ones proposed by Tzeng and Hu.Category / Keywords: Visual cryptography, Pixel expansion. Date: received 16 May 2006 Contact author: carblu at dia unisa it Available formats: PDF | BibTeX Citation Version: 20060516:193315 (All versions of this report) Discussion forum: Show discussion | Start new discussion