A DCT Domain VisibleWatermarking Technique for Images

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INTRODUCTION
Digital watermarking is defined as a process of embedding data
(watermark) into a multimedia object to help to protect the
owner's right to that object. The embedded data (watermark) may
be either visible or invisible.
In visible watermarking of images, a secondary image (the
watermark) is embedded in a primary (host) image such that
watermark is intentionally perceptible to a human observer
whereas in the case of invisible watermarking the embedded data
is not perceptible, but may be extracted/detected by a computer
program.



2. FINDING THE SCALING AND EMBEDDING FACTORS
While finding the scaling factors (an) and embedding factors
(bn), the following are taken into consideration [4][5][6][7] so
that the quality of the watermarked image is not degraded.
· The edge blocks should be least altered to avoid significant
distortion of the image. So one can add only small amount
of watermark gray value in the edge block of host image.
This means that scaling factor an should be close to amax ,
(the maximum value of the scaling factor) and embedding
factor bn should be close to bmin, (the minimum value of the
embedding factor).
· The distortion visibility is low when the background has
strong texture. In a highly textured block, energy tends to be
more evenly distributed among the different AC DCT
coefficients. That means AC DCT coefficients of highly
textured blocks have small variances and we can add more
to those blocks. So for convenience, we assume an to be
directly proportional to variance (sn) and bn to be inversely
proportional to variance (sn).


3. INSERTION OFWATERMARK
Figure 5 gives the schematic representation of the insertion
process.
The steps for watermark insertion are discussed now.
· The original image I (to be watermarked) and the watermark
image W are divided into blocks of size 8x8. (Both the
images may not be of equal size).
· The DCT coefficients for each block of the original image
are found out.
· For each block of the original image I, the normalized mean
gray value m’
n is computed using eqn.(4) and are scaled to
the range 0.1-1.0.



MODIFICATIONS TO MAKE THEWATERMARK MORE ROBUST

The algorithm proposed here and also that of the classification
schemes proposed in [4] are not robust for images having very
few objects and large uniform areas like in Fig.9 ('hardware
image'). In [4] most of the blocks will be classified to be in one
class for this type of image. If the algorithm discussed in Section
3 is applied then most of the blocks will have the same an and bn
values as is clear from Fig.10-Fig.11. The different an and bn are
sorted and displayed here to get a clear understanding of the
situation. So in either of the cases, it is easy for a digital thief to
remove the watermark from the watermarked image as it would
be easy to predict the an and bn values.