Image Watermarking
Feng CHEN
02430339
159.731 Machine Vision Seminar Report
June 2004
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Table of Contents
1. Introduction
3
2.
Motivation
4
3.
Approaches of Watermarking
5
4.
Multi-resolution watermarking
6
5.
Applications of watermarking
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6.
Requirements of watermarking
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7.
Conclusions
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8.
References
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1. Introduction
The rapid expansion of the Internet in the past years has rapidly increased the
availability of digital data such as audio, images and videos to the public. As we have
witnessed in the past few months, the problem of protecting multimedia information
becomes more and more important and a lot of copyright owners are concerned about
protecting any illegal duplication of their data or work. Some serious work needs to
be done in order to maintain the availability of multimedia information but, in the
meantime, the industry must come up with ways to protect intellectual property of
creators, distributors or simple owners of such data. This is an interesting challenge
and this is probably why so much attention has been drawn toward the development
of digital images protection schemes. Of the many approaches possible to protect
visual data, digital watermarking is probably the one that has received most interest
The idea of robust watermarking of images is to embed information data within the
image with an insensible form for human visual system but in a way that protects
from attacks such as common image processing operations.
As the computers are more and more integrated via the network, the distribution of
digital media is becoming faster, easier, and requiring less effort to make exact copies.
One of the major impediments is the lack of effective intellectual property protection
of digital media to discourage unauthorized copying and distribution.
1.1 What is a watermark?
A watermark is the hidden information within a digital signal (such as image, video,
audio, polygonal model...).It is integrated into the content of host signal itself, and
requires no additional file header or conversion of data format as well. Moreover, it is
designed to permanently reside in the host data .Finally, unlike encryption; it does not
restrict access to the host data.
1.2 What is a digital watermarking?
Also referred to as simply watermarking, a pattern of bits inserted into a digital image,
audio or video file that identifies the file's copyright information (author, rights, etc.).
The name comes from the faintly visible watermarks imprinted on stationery that
identify the manufacturer of the stationery.
The purpose of digital watermarks is to provide copyright protection for intellectual
property that's in digital format.
Unlike printed watermarks, which are intended to be somewhat visible, digital
watermarks are designed to be completely invisible, or in the case of audio clips,
inaudible. Moreover, the actual bits representing the watermark must be scattered
throughout the file in such a way that they cannot be identified and manipulated. And
finally, the digital watermark must be robust enough so that it can withstand normal
changes to the file, such as reductions from lossy compression algorithms.
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Satisfying all these requirements is no easy feat, but there are a number of companies
offering competing technologies. All of them work by making the watermark appear
as noise - that is, random data that exists in most digital files anyway. To view a
watermark, you need a special program that knows how to extract the watermark data.
Watermarking is also called data embedding and information hiding.
2. Motivation
2.1 In the past
Conventionally, in analog world, a painting is signed by the artist to attest the
copyright, an identity card is stamped by the steel seal to avoid forgery, and the paper
money are identified by the embossed portrait. Such kind of hand-written signatures,
seals and watermarks have been used from ancient times as a way to identify the
source, creator of a document or a picture. For example, a priceless painting of the
11th century in National Palace Museum named "Travelers on a Mountain Path" had
not been identified as the genuine work of Fan Kuan until Fan's signature is found
between the woods behind a group of travelers of the painting.
Travelers on a Mountain
Figure 1
Fan’s signature “范宽”
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2.2 In digital World
However, in the digital world, digital technology for manipulating images has make it
difficult to distinguish the visual truth. Besides, the characteristics of digitization
bring significant hangs in copyright issues, which create an urgent need to intellectual
property protection on the digitally recorded information.
Digital watermarking has been proposed as a way to claim the ownership of the
source and owner. Unlike encryption, watermarking does not restrict access to the
data. Once the encrypted data is decrypted, the intellectual property rights are no
longer protected.
Over the past few years, the technology of the digital watermarking has gained
prominence and emerged as a leading candidate that could solve the fundamental
problems of legal ownership and content authentications for digital multimedia data.
A great deal of research efforts has been focused on digital image watermarking in
recent years. The techniques proposed so far can be divided into two groups according
to the embedding domain.
One group is spatial domain approach.
The other group is frequency domain approach.
3. Approaches of watermarking
3.1 Spatial Domain Approach
The earliest watermarking techniques are mainly this kind and the simplest example is
to embed the watermark into least significant bits (LSBs) of the image pixels.
However, this technique has relatively low information hiding capacity and can be
easily erased by lossy image compression.
3.2 Frequency Domain Approach
Another way to produce high quality watermarked image is by first transforming the
original image into the frequency domain by the use of Fourier, Discrete Cosine or
Wavelet transforms for example. And it can embed more information bits and is
relatively robust to attack. With this technique, the marks are not added to the
intensities of the image but to the values of its transform coefficients. Then
inverse-transforming the marked coefficient forms the watermarked image. The use of
frequency based transforms allows the direct understanding of the content of the
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image; therefore, characteristics of the human visual system (HVS) can be taken into
account more easily when it is time to decide the intensity and position of the
watermarks to be applied to a given image.
3.2.1 Examples of Implementation
1. Cox et al. used the spread spectrum communication for digital multimedia
watermark.
2. Hsu and Wu embedded an image watermark into selectively modified middle
frequency of discrete cosine transform (DCT) coefficients of container image.
3. Joseph et al. developed a digital image watermarking using the Fourier-Mellin
transform that is invariant to image manipulations or attacks due to rotation, scaling
and translation.
Several other methods used discrete wavelet transform to hide the data to the
frequency domain.
4. Multi-resolution Watermarking
4.1 Description
Here, I am going to discuss an example on watermarking method proposed by
Ward .This first generation technique called Multi-resolution Watermark for Digital
Images works in the wavelet domain.
The modus operandi is pretty simple since it consists of adding weighted
pseudo-random codes to the large coefficients at the high and medium frequency
bands of the discrete wavelet transform of an image. Even if it might first appear
theoretically straightforward the actual implementation is a little more complicated
than it seems and great effort will be made here to clarify it.
First of all, one must know why the wavelets approach is used here. The
characteristics of this transform domain are well suited for masking consideration
since it is well localized both in time and frequency. Besides wavelet transforms
match the multi channel model of the HVS so one can actually set a numerical limit to
the wavelets coefficients alteration in order to stay under the HVS just noticeable
difference 3 (JND), for which our eyes start to become aware of the modifications in
the image . Besides, wavelet transforms is a part of upcoming compression standards
(such as JPEG-2000) so wavelet-based techniques would allow a much easier and
optimized way to include a copyright protection device in the compression code itself.
A second important aspect of the technique used here is the way to introduce a
watermark in an image. A watermark should be introduced in perceptually
significant regions of the data in order to remain robust. However, by doing so,
we risk to alter the image (i.e. perceivably). The technique described here
follows to a certain degree this requirement but tries to make the introduced
watermark as invisible as it can while showing good robustness. As the reader
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will understand, we have chosen to ensure transparency of the watermark and,
in the same time keep the robustness by embedding the information within only
the high and medium frequencies while keeping third resolution low
frequencies, for which the HVS is most sensitive, untouched. The implemented
technique uses no oblivious watermark. In copyright protection, it is reasonable
to assume that the owner or the user who wants to verify the presence of a
particular watermarking key has access to the original un-watermarked image.
This type of watermarking scheme is called non-oblivious (or private)
watermarking in opposition to oblivious (or public) techniques, such as copy
protection, where the original image is not accessible.
4.2 Steps of Implementation
4.2.1 Encoder
The first part of the watermarking process is, of course, the encoder. The first step is
to decompose the image into ten frequency bands using three resolutions of Haar
wavelets.
Figure 2’s bank of filters represents well the idea of the octave-band structure of Haar
wavelets, which gives pyramid structure frequency localization as shown in Figure 3.
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Here, I must point out that an under sampling operation is done after every filtering. It
must be understood that the choice of the Haar wavelet in our system was one made
for simplicity. However, we had in mind to investigate the influence of the choice of
the wavelet function in our results but, in order to test the robustness truthfully, we
had to give up the idea in favor of the addition of extra robustness testing procedures.
The next operation is to add a pseudo random sequence N , in fact a Gaussian
distribution of mean zero and variance one, to the coefficients of the medium and high
frequency bands (i.e. all the bands except the lowest one which is represented by the
top left corner in Figure 3). The normal distribution is used because it has been
proven to be quite robust to collusive attacks. In order to weight the watermark
according to the magnitude of the wavelet coefficients, we used one of the two
following relations between the original coefficients y and ÿ , the ones containing the
watermark:
ÿ[m,n] = y[m,n] + alpha (y[m,n])2 N[m,n]
&
ÿ[m,n] = y[m,n] + alpha . abs(y[m,n]) N[m,n]
(1)
(2)
It must be pointed out that the relations (1) and (2), even though they are
mathematically different, have the exact same goal which is to put more weight to the
watermark added to high value wavelet coefficients. The parameter alpha is to control
the level of the watermark; it is in fact a good way to choose between good
transparency or good robustness or a tradeoff between the two. Finally, the
two-dimension inverse wavelet transform of is computed to form ÿ the watermarked
image.
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Figure 4 gives a good idea of the main components of the encoder
4.2.2 Decoder
At the other end of the communication channel, a decoder is used to extract the
watermarked information from the received image. Upon reception of the supposedly
watermarked image, the algorithm first isolates the signature included in this image
by comparing the DWT coefficients of the image with those of the original
(non-watermarked) one. The following operation consists of taking the identified key
to put in contrast with the found signature by computing the cross-correlation at the
first resolution level (i.e. highest frequency coefficients). The watermark is called
detected if there is a peak in the cross-correlation corresponding to a positive
identification. If there is no central peak, the decoder adds the second resolution level
(i.e. the bottom left square in the pyramid structure of Figure 3) to the computation
aiming at finding for a peak. Once again, if there is a peak, the watermark is called
detected and if not, we go to the third resolution… and so on until we reach the ninth
resolution limit.
4.3 Advantage of this technique
The main advantage of this technique is that while allowing good detection, even in
the presence of corruption, it keeps the level of false positive detection to a minimum
since the found signature has to go through detection step of positive identification to
be called detected. The detector step aims at ensuring the maximum exactitude in the
detection of the owner identification key and, as said previously, minimizing the
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number of false positive detection. The results presented later on should convince the
reader of the performance of our decoder.
5. Applications of Watermarking
5.1 Fingerprinting
In order to trace the source of illegal copies the owner can embed different
watermarking keys in the copies that are supplied to different customer. For the owner,
embedding a unique serial number-like watermark is a good way to detect customers
who break their license agreement by copying the protected data and supplying it to a
third party.
5.2 Indexing
Watermarking offers a wide range of new capabilities to multimedia applications. It
allows the indexing of video mail by permitting the insertion of comments in video
content as well as the indexing of movies or news items by making available the
utilization of markers that can be exploited in search engines. As the number of
images and video contents online increases a lot faster than the capabilities of today’s
search engine, it is important to plan ahead for new ways to allow quick access to
multimedia data and watermarking is certainly a promising way to do so.
5.3 Copyright Protection & Owner identification:
To protect its intellectual property, the data owner can embed a watermark
representing copyright information of his data. This application can be a really helpful
tool in settling copyright disputes in court. It is probably the most widely spread use
of digital images watermarking and it is also the application we have worked on in the
present project.
5.4 Broadcast monitoring:
In order to help the automated identification of broadcasted programs, original
watermarks can be inserted in any type of data to be widely broadcasted on a network.
It could assure that advertisers received the airtime they have paid for or make certain
that musicians’ property is not rebroadcast by pirate stations (or at least, if so, that it
can be detected).
5.5 Copy protection:
The watermarked information can directly control digital recording device. The
embedded key can represent a copy-permission bit stream that is detected by the
recording device which then decide if the copying procedure should go on (allowed)
or not (prohibited).
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5.6 Data Authentication:
Fragile watermarks are used to detect any corruption of an image or any other type of
data. If the watermark is detected, the data is genuine, if not, the data has been
corrupted and cannot be considered.
5.7 Data Hiding (Covert Communications):
The transmission of private data is probably one of the earliest applications of
watermarking. As one would probably have already understood, it consists of
implanting a strategic message into an innocuous one in a way that would prevent any
unauthorized person to detect it.
5.8 Medical Safety:
Embedding the date and patient’s name in medical images could increase the
confidentiality of medical information as well as the security.
6. Requirements of Watermarking
To achieve maximum protection of intellectual property with watermarked media,
several requirements must be satisfied:
6.1 Imperceptible
The watermark should be imperceptible so as not to affect the viewing experience of
the image or the quality of the audio signal.
6.2 Undeletable
The watermark must be difficult or even impossible to remove by a hacker, at least
without obviously degrading the host signal.
6.3 Statistically Undetectable
A pirate should not be able to detect the watermark by comparing several
watermarked signals belonging to the same author.
6.4 Robustness
6.4.1 The watermark should be survive the lossy compression techniques like JPEG,
which is commonly used for transmission and storage.
6.4.2 The watermark should still be retrievable even if common signal processing
operation are applied, such as signal enhancement, geometric image operations, noise,
filtering, and etc.
6.5 Unambiguous
Retrieval of the watermark should be unambiguously identify the owner, and the
accuracy of identification should degrade gracefully in the face of attacks
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7. Conclusions
As we have witnessed in the past few months, the problem of protecting
multimedia information becomes more and more important and a lot of
copyright owners are concerned about protecting any illegal duplication of their
data or work. Some serious work needs to be done in order to maintain the
availability of multimedia information but, in the meantime, the industry must
come up with ways to protect intellectual property of creators, distributors or
simple owners of such data.
This is an interesting challenge and this is probably why so much attention has
been drawn toward the development of digital images protection schemes.
Of the many approaches possible to protect visual data, digital watermarking is
probably the one that has received most interest. The goal of digital
watermarking is to produce an image that looks exactly the same to a human
eye but still allows its positive identification in comparison with the owner's
key if necessary.
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8. References
[1] I J Cox , M L Miller and J A Bloom , Watermarking Applications and
their Properties .
[2] A H Paquet , R K Ward , Wavelet Based Digital Watermarking
[3] G Voyatzis and I Pitas , The Use Of Watermarks in the Protection of
Digital Multimedia Products .
[4] H Y Lo , S Topiwala and J Wang , Wavelet Based Steganography and
Watermarking .
[5] http://www.webopedia.com/TERM/D/digital_watermark.html
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