Cryptography Reference
In-Depth Information
(a)
(b)
(c)
Fig. 11.5.
Original image and watermarks. (a) Original Lena image. (b) Original
robust watermark. (c) Original semi-fragile watermark.
(a)
(b)
Fig. 11.6.
The watermarked images obtained by algorithms having different em-
bedding orders. (a) The watermarked image with PSNR = 30.553 dB obtained by
using the proposed method. (b) The watermarked image with PSNR = 26.105 dB
obtained by using the algorithm in the reverse embedding order.
image quality than the alternative. The first reason for this is that the first-
stage codebook is small. The second reason is that the robust embedding
algorithm [20] does not modify the encoded indices at all. In contrast the
semi-fragile watermarking method does. If we embed the semi-fragile water-
mark in the first stage, then the reconstructed error image may be very poor,
which greatly affects the reconstructed image.
In this chapter, we use the
Normalized Hamming Similarity
, NHS, to eval-
uate the effectiveness of the proposed algorithm. The NHS between the em-
bedded binary watermark W and that extracted W
′
is defined as
′
NHS = 1−
HD (W, W
)
,
(11.13)
A
w
B
w
where HD(,) denotes the Hamming Distance between two binary strings.
That is, the number of different bits in the two binary strings. We can easily
