Visual Cryptography from Halftone Error Diffusion - Visual Cryptography and Secret Image Sharing - page 20

shares and decoding of the secret image can be performed in a digital way
where the decoding rule remains the same (OR operation). The human visual
system is still the ultimate tool to identify the secret image.
The robustness of the introduced scheme to the contrast loss is attributed
to the fact that shares have well-defined local structure. If decoded digitally,
we can measure the local intensity of the reconstructed image by filtering
the data through a running window. Then we measure the local intensity of
the data within the window. We can assign 1 to the current pixel if the local
intensity is relatively high and assign 0 to the current pixel if the local intensity
is relatively low. By such a simple method, the contrast of the reconstructed
image can be enhanced and the content is more readable. The size of the
running window should be the same as the size of the halftone cell. However,
if the halftone cell size is unknown, a proper window size can be obtained by
trying different window sizes until only two different local intensities are most
likely to appear.

1.7 Simulation Results

(a) (b)

FIGURE 1.9
(a) Grayscale image Lena. (b) Part of the distribution of SIPs and ABPs.
The gray pixels indicate SIPs and the black pixels indicate ABPs. (Reprinted
with permission from IEEE Trans. Inf. Forensics Security, vol. 4, no. 3, pp.
383{396, Sep. 2009 c IEEE 2009).