01-12-2012, 02:34 PM
A DCT Domain VisibleWatermarking Technique for Images
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ABSTRACT
The growth of computer networks has boosted the growth of the
information technology sector to a greater extent. There is a trend
to move from conventional libraries to digital libraries. In the
digital libraries images and texts are made available through the
internet for scholarly research. At the same time care is taken to
prevent the unauthorized use of the images commercially. In
some cases the observer is encouraged to patronize the institution
that owns the material. To satisfy both these needs
simultaneously the owner needs to use visible watermarking.
Visible watermarking is a type of digital watermarking used for
protection of publicly available images. In this paper, we
describe a visible watermarking scheme that is applied into the
host image in the DCT domain. A mathematical model has been
developed for that purpose. We have also proposed a
modification of the algorithm to make the watermark more
robust.
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.
Some of the desired characteristics of visible watermarks are
listed below [1][2].
· A visible watermark should be obvious in both color and
monochrome images.
· The watermark should be spread in a large or important area
of the image in order to prevent its deletion by clipping.
· The watermark should be visible yet must not significantly
obscure the image details beneath it.
· The watermark must be difficult to remove; removing a
watermark should be more costly and labor intensive than
purchasing the image from the owner.
· The watermark should be applied automatically with little
human intervention and labor.
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).
MODIFICATIONS TO MAKE THE
WATERMARK 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.
CONCLUSIONS
A visible watermarking technique has been proposed in the DCT
domain. A mathematical model has been developed for this
purpose exploiting the texture sensitivity of the HVS. We have
also proposed a modification to increase the robustness of the
watermark when used for images with very few objects. For more
robustness, the watermark should not be made publicly available,
the watermark should be used in different sizes and should be put
in different portions for different images. We have used lower
values of amin and amax , and higher values of bmin and bmax to
make the watermark more prominent even when the images are
printed on paper.