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Figure 11.10 (a{c) reveal the recovered secret images for 2 = 4, 16, and
64, respectively. The clearness of the recovered secret image diminishes when
the misalignment increases. The misalignment tolerant VCS using 2 = 64
has the best misalignment tolerant capability, whereas it has the worst visual
quality for no deviation. On the contrary, the misalignment tolerant VCS
using 2 = 4 has a different characteristic. We may trade the misalignment
tolerance for the image quality by the size ratio.
11.6 Conclusions and Discussions
This chapter is intended to show some recent results about the alignment
problem of the VCS. We considered two kinds of misalignment, (1) misalign-
ment with integer number of subpixels and (2) misalignment with less than
one subpixel. In both cases, the secret image can be visually recovered. This
phenomenon indicates that, the VCS, by itself, has some misalignment toler-
ance.
Then we compared the misalignment tolerance of large and small subpix-
els, and showed that the large subpixel had better misalignment tolerance.
Based on this result, a misalignment tolerant VCS was given that traded the
large and small subpixels in one share. Simulations were provided to show the
performance of the misalignment tolerant VCS.
Because of the page limit, we cannot introduce more misalignment toler-
ant VCS. Interested readers can find more information about the alignment
problem of the VCS in the following papers [8, 16, 13, 6, 9].
11.7 Acknowledgments
I gratefully acknowledge Prof. Chuankun Wu, as he introduced me to visual
cryptography. We shared many pleasant memories on studying visual cryp-
tography. This work was supported by NSFC grant No. 60903210.
Bibliography
[1] G. Ateniese, C. Blundo, A. De Santis, and D.R. Stinson. Visual cryp-
tography for general access structures. In Information and Computation,
volume 129, pages 86{106, 1996.
 
 
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