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.
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.
2.1In 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”
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.
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.
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 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.
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.
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 operandiis 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 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
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.