Image Watermarking

1. 
   Introduction 3
   

2. 
   Motivation 4
   

3. 
   Approaches of Watermarking 5
   

4. 
   Multi-resolution watermarking 6
   

5. 
   Applications of watermarking 10
   

6. 
   Requirements of watermarking 11
   

7. 
   Conclusions 12
   

1. 
   Introduction
   

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.

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.

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. 
   Motivation
   

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”

2. 
   In digital World
   

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.

2. 
   Approaches of watermarking
   

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.

2. 
   Frequency Domain Approach
   

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.

2. 
   Multi-resolution Watermarking
   

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.

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.