Zhou Zhi, Arce Gonzalo R, Di Crescenzo Giovanni
Department of Electrical and Computer Engineering, University of Delaware, Newark, DE 19716, USA.
IEEE Trans Image Process. 2006 Aug;15(8):2441-53. doi: 10.1109/tip.2006.875249.
Visual cryptography encodes a secret binary image (SI) into n shares of random binary patterns. If the shares are xeroxed onto transparencies, the secret image can be visually decoded by superimposing a qualified subset of transparencies, but no secret information can be obtained from the superposition of a forbidden subset. The binary patterns of the n shares, however, have no visual meaning and hinder the objectives of visual cryptography. Extended visual cryptography [1] was proposed recently to construct meaningful binary images as shares using hypergraph colourings, but the visual quality is poor. In this paper, a novel technique named halftone visual cryptography is proposed to achieve visual cryptography via halftoning. Based on the blue-noise dithering principles, the proposed method utilizes the void and cluster algorithm [2] to encode a secret binary image into n halftone shares (images) carrying significant visual information. The simulation shows that the visual quality of the obtained halftone shares are observably better than that attained by any available visual cryptography method known to date.
可视密码术将一个秘密二进制图像（SI）编码为n个随机二进制图案的份额。如果将这些份额复印到透明片上，通过叠加一组合格的透明片，就可以直观地解码出秘密图像，但是从一组被禁止的透明片叠加中无法获取任何秘密信息。然而，这n个份额的二进制图案没有视觉意义，阻碍了可视密码术的目标实现。最近提出了扩展可视密码术[1]，利用超图着色来构造有意义的二进制图像作为份额，但视觉质量较差。本文提出了一种名为半色调可视密码术的新技术，通过半色调处理来实现可视密码术。基于蓝噪声抖动原理，该方法利用空洞和聚类算法[2]将一个秘密二进制图像编码为n个携带重要视觉信息的半色调份额（图像）。仿真结果表明，所得到的半色调份额的视觉质量明显优于目前已知的任何可视密码术方法所获得的视觉质量。
