Cryptography Reference
In-Depth Information
FIGURE 13.7
Image Y obtained by overlaying Y 1 in Figure 13.4 and Y 2 in Figure 13.6.
W, Figure 13.4 is Y 1 and Figure 13.6 is Y 2 generated using DHSED with
respect to Y 1 and W. A threshold L = 5 is used. Note that Figure 13.6
looks like Figure 13.4, which verifies that DHSED can give halftone images
with good visual quality. Figure 13.7 shows the image obtained by overlaying
Figure 13.6 and Figure 13.4. The secret pattern W is clearly visible in Figure
13.7 verifying that DHSED is an effective visual cryptography method.
13.4 Data Hiding by Conjugate Error Diffusion (DHCED)
DHSED as outlined in the previous section is both computationally and con-
ceptually simple, but suffers from three major problems. First, Y 1 and Y 2
must be obtained from the same X. In other words, DHSED cannot embed
a binary secret pattern in two halftone images obtained from two different
multitone images. Second, when the images are overlaid, the contrast of the
revealed secret pattern is relatively low. Third, occasional boundary artifacts
may happen in Y 2 especially towards the right and bottom sides of the secret
pattern where error is not diffused properly across the W b boundary. Such
boundary artifacts can lower the visual quality of Y 2 considerably.
In this section, we will introduce another method called Data Hiding by
Conjugate Error Diffusion (DHCED) that addresses these three problems.
 
 
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