Cryptography Reference
In-Depth Information
In embedding the watermark, if the codeword for the current block is
c
k
,k∈[0,L−1], and if the watermark bit is 0, C
e
is adopted for water-
mark embedding because 0 is an even number, and the nearest codeword
in C
e
is found to replace the original codeword. If the watermark bit is
1, C
o
is used, and the same scheme is applied for embedding watermark
bit 1. Finally, with the codewords in C
o
and C
e
, the watermarked image
X
′
is reconstructed.
In extracting the watermark, the codebook plays an essential role. Sup-
pose that the watermarked image X
′
is transmitted over the packet loss
channel, and the received image is denoted by X
′′
. In the receiver side, the
same sub-codebooks C
o
and C
e
are employed to extract the watermark.
The authors used table look-up to find the VQ indices of the received
image X
′′
. For every block in vector quantization, if the index belongs to
C
o
, then the extracted watermark bit is determined to be 0; if not, the
bit is 1. By gathering all the extracted watermark bits, the extracted
watermark W
′
can be obtained.
12.9 Simulation Results
In our simulations, we take the test image,
Lena
, with size 512512, as
the original source. We have the embedded watermarks with size 128128,
shown in Fig. 12.8. For embedding a single watermark, we employ watermark
1 only; for embedding multiple watermarks, we employ both the watermarks.
The original source is divided into 44 blocks for VQ compression, which also
meets the number of bits required for watermark embedding. The codebook
sizes are L = 512 and L = 1024, and indices therein are represented by 9-bit
or 10-bit strings, respectively.
watermark 1
watermark 2
Fig. 12.8.
The two watermarks used for embedding in this chapter, both have sizes
128128. Watermark 1 shows a flower, while watermark 2 denotes the characters
KUAS, representing the authors a
liation.
Two quantities are considered for evaluating the proposed algorithms. We
employ the watermarked image quality as the first metric for evaluation. The
