Improving the visual quality of size invariant visual cryptography scheme

  • Authors:
  • Feng Liu;Teng guo;ChuanKun Wu;Lina Qian

  • Affiliations:
  • State Key Laboratory Of Information Security, Institute of Information Engineering, Chinese Academy of Sciences, China;State Key Laboratory Of Information Security, Institute of Information Engineering, Chinese Academy of Sciences, China and Graduate School of Chinese Academy of Sciences, Beijing 100190, China;State Key Laboratory Of Information Security, Institute of Information Engineering, Chinese Academy of Sciences, China;State Key Laboratory Of Information Security, Institute of Information Engineering, Chinese Academy of Sciences, China

  • Venue:
  • Journal of Visual Communication and Image Representation
  • Year:
  • 2012

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Abstract

In order to reduce the pixel expansion of visual cryptography scheme (VCS), many size invariant visual cryptography schemes (SIVCS's) were proposed. However, most of the known SIVCS's have bad visual quality and thin line problems, hence the known SIVCS's are only suitable to encrypt coarse secret images. In this paper, we notice that the variance of the darkness levels of the pixels also reflects the visual quality of the recovered secret image, as well as the average contrast. We verify, analytically and experimentally, the effectiveness of the variance to be a criterion for evaluating the visual quality of the recovered secret image. Furthermore, we propose two multi-pixel encryption size invariant visual cryptography schemes (ME-SIVCS's) which improve the visual quality of the recovered secret image by reducing the variance of the darkness levels. In addition, the proposed ME-SIVCS's can be used to encrypt fine secret images since they avoid some known thin line problems. Experimental results and comparisons are also given to show the effectiveness of the proposed ME-SIVCS's. Finally, we give suggestions on obtaining good visual quality for the recovered secret image.