Choice of location for the watermark bits in the cover
The choice of the location of the watermark is crucial for both security and psychovisual impact
of the watermarked image. I explore some of the methods related to the choie of location.
Patchwork Algorithm
Suggested by Bender et al.(1995) this asks the binary question “does the person know the key used to build watermark”
The insertion process is done as follows: the sender generates using a PRNG n random locations in the image (a(i), b(i) for
i = 1 to n (where n is the length of the watermark). Now a(i) = a(i) + 1 ; b(i) = b(i) - 1.
The extraction process is to use the secret key on hand to get the random locations and then
sum the values at those locations
Iff the secret key were the right one the sum would approximately be equal
to 2*n else it would be approximately equaly to zero.
This is due to the statistical property:
E(s) = sum(E[a(i)] - E[b(i)]) = 0 (for i = 1..n)
Public-key crypto and public-key watermarking
The big drawback with most watermarking methods is the shared-key concept. Ideally the public
key crypto concept can be applied to watermarking wherein there are two keys : one of them
the private key can be used to watermark the image and the public key to verify it.
Hartung and Girod[18] show how to use public-key techniques using direct seqeunce spread-spectrum
models. The direct sequence technique usually requires the spread sequence S for both spreading
and unspreading; however due to the robustness of the encoding it is possible to reconstruct
the original key without the knowledge of the entire sequence.
Predictive coding for psychovisual watermarking
These are widely used in source coding to predict new value from former value. The assumption
is that neighbouring pixels are highly correlated. In the watermarking context predictive coding
is useful for its pschovisual effectivness. The human vision system is less accurate in
textured/edge regions. However its very sensitive to smooth regions with uniform values; in
predictive coding the error conditions matche these properties and the error signal can be
used as a modulation carrier for the watermark. It also is in the literature[19] that predictive
coding does not need the original image