In recent years, the distribution of works of art, including pictures, music, video and textual documents, has become easier. With the widespread and increasing use of the Internet, digital forms of these media (still images, audio, video, text) are easily accessible. This is clearly advantageous, in that it is easier to market and sell one's works of art. However, this same property threatens copyright protection. Digital documents are easy to copy and distribute, allowing for pirating. There are a number of methods for protecting ownership. One of these is known as digital watermarking. Digital watermarking is the process of inserting a digital signal or pattern (indicative of the owner of the content) into digital content. The signal, known as a watermark, can be used later to identify the owner of the work, to authenticate the content, and to trace illegal copies of the work.
Watermarks of varying degrees of obtrusiveness are added to presentation media as a guarantee of authenticity, quality, ownership, and source.
To be effective in its purpose, a watermark should adhere to a few requirements. In particular, it should be robust, and transparent. Robustness requires that it be able to survive any alterations or distortions that the watermarked content may undergo, including intentional attacks to remove the watermark, and common signal processing alterations used to make the data more efficient to store and transmit. This is so that afterwards, the owner can still be identified. Transparency requires a watermark to be imperceptible so that it does not affect the quality of the content, and makes detection, and therefore removal, by pirates less possible.
The media of focus in this paper is the still image. There are a variety of image watermarking techniques, falling into 2 main categories, depending on in which domain the watermark is constructed: the spatial domain (producing spatial watermarks) and the frequency domain (producing spectral watermarks). The effectiveness of a watermark is improved when the technique exploits known properties of the human visual system. These are known as perceptually based watermarking techniques. Within this category, the class of image-adaptive watermarks proves most effective.
In conclusion, image watermarking techniques that take advantage of properties of the human visual system, and the characteristics of the image create the most robust and transparent watermarks.
1.1 THE DIGITAL WATERMARK
Digital watermarking is a technology for embedding various types of information in digital content. In general, information for protecting copyrights and proving the validity of data is embedded as a watermark.
A digital watermark is a digital signal or pattern inserted into digital content. The digital content could be a still image, an audio clip, a video clip, a text document, or some form of digital data that the creator or owner would like to protect. The main purpose of the watermark is to identify who the owner of the digital data is, but it can also identify the intended recipient.
Why do we need to embed such information in digital content using digital watermark technology? The Internet boom is one of the reasons. It has become easy to connect to the Internet from home computers and obtain or provide various information using the World Wide Web (WWW).
All the information handled on the Internet is provided as digital content. Such digital content can be easily copied in a way that makes the new file indistinguishable from the original. Then the content can be reproduced in large quantities.
For example, if paper bank notes or stock certificates could be easily copied and used, trust in their authenticity would greatly be reduced, resulting in a big loss. To prevent this, currencies and stock certificates contain watermarks. These watermarks are one of the methods for preventing counterfeit and illegal use.
Digital watermarks apply a similar method to digital content. Watermarked content can prove its origin, thereby protecting copyright. A watermark also discourages piracy by silently and psychologically deterring criminals from making illegal copies.
Principle of digital watermarks
A watermark on a bank note has a different transparency than the rest of the note when a light is shined on it. However, this method is useless in the digital world.
Currently there are various techniques for embedding digital watermarks. Basically, they all digitally write desired information directly onto images or audio data in such a manner that the images or audio data are not damaged. Embedding a watermark should not result in a significant increase or reduction in the original data.
Digital watermarks are added to images or audio data in such a way that they are invisible or inaudible Ñ unidentifiable by human eye or ear. Furthermore, they can be embedded in content with a variety of file formats. Digital watermarking is the content protection method for the multimedia era.
Materials suitable for watermarking
Digital watermarking is applicable to any type of digital content, including still images, animation, and audio data. It is easy to embed watermarks in material that has a comparatively high redundancy level ("wasted"), such as color still images, animation, and audio data; however, it is difficult to embed watermarks in material with a low redundancy level, such as black-and-white still images.
To solve this problem, we developed a technique for embedding digital watermarks in black-and-white still images and a software application that can effectively embed and detect digital watermarks.
Structure of a digital watermark
The material that contains a digital watermark is called a carrier. A digital watermark is not provided as a separate file or a link. It is information that is directly embedded in the carrier file. Therefore, simply viewing the carrier image containing it cannot identify the digital watermark. Special software is needed to embed and detect such digital watermarks. Kowa 's SteganoSign is one of these software packages.
Both images and audio data can carry watermarks.
1.2 THE IMPORTANCE OF DIGITAL WATERMARKS
The Internet has provided worldwide publishing opportunities to creators of various works, including writers, photographers, musicians and artists. However, these same opportunities provide ease of access to these works, which has resulted in pirating. It is easy to duplicate audio and visual files, and is therefore probable that duplication on the Internet occurs without the rightful owners' permission.
An example of an area where copyright protection needs to be enforced is in the on-line music industry. The Recording Industry Association of America (RIAA) says that the value of illegal copies of music that are distributed over the Internet could reach $2 billion a year.
Digital watermarking is being recognized as a way for improving this situation. RIAA reports that "record labels see watermarking as a crucial piece of the copy protection system, whether their music is released over the Internet or on DVD-Audio". They are of the opinion that any encryption system can be broken, sooner or later, and that digital watermarking is needed to indicate who the culprit is.
Another scenario in which the enforcement of copyright is needed is in newsgathering. When digital cameras are used to snapshot an event, the images must be watermarked as they are captured. This is so that later, image's origin and content can be verified. This suggests that there are many applications that could require image watermarking, including Internet imaging, digital libraries, digital cameras, medical imaging, image and video databases, surveillance imaging, video-on-demand systems, and satellite-delivered video.
1.3 THE PURPOSES OF DIGITAL WATERMARKS
Watermarks are a way of dealing with the problems mentioned above by providing a number of services:
1. They aim to mark digital data permanently and unalterably, so that the source as well as the intended recipient of the digital work is known. Copyright owners can incorporate identifying information into their work. That is, watermarks are used in the protection of ownership. The presence of a watermark in a work suspected of having been copied can prove that it has been copied.
2. By indicating the owner of the work, they demonstrate the quality and assure the authenticity of the work.
3. With a tracking service, owners are able to find illegal copies of their work on the Internet. In addition, because each purchaser of the data has a unique watermark embedded in his/her copy, any unauthorized copies that s/he has distributed can be traced back to him/her.
4. Watermarks can be used to identify any changes that have been made to the watermarked data.
5. Some more recent techniques are able to correct the alteration as well.
1.4 OVERVIEW OF COPYRIGHT LAW
"In essence, copyright is the right of an author to control the reproduction of his intellectual creation" . When a person reproduces a work that has been copyrighted, without the permission of the owner, s/he may be held liable for copyright infringement. To prove copyright infringement, a copyright owner needs to prove 2 things.
1. S/he owns the copyright in the work, and
2. The other party copied the work (usually determined by establishing that the other party had access to the copyrighted work, and that the copy is "substantially similar" to the original).
In cases where it cannot be said that the owner's work and the possible illegal copy are identical, the existence of a digital watermark could prove guilt.
The damages charge can be higher if it can be proven that the party's conduct constitutes willful infringement; that is, s/he copied the work even though s/he knew that it was copyrighted (for example, copying even after having discovered a watermark in the work).
2. DIGITAL WATERMARK TYPES AND TERMS
Watermarks can be visible or invisible:
a. Visible watermarks are designed to be easily perceived by a viewer (or listener). They clearly identify the owner of the digital data, but should not detract from the content of the data.
b. Invisible watermarks are designed to be imperceptible under normal viewing (or listening) conditions; more of the current research focuses on this type of watermark than the visible type.
Both of these types of watermarks are useful in deterring theft, but they achieve this in different ways. Visible watermarks give an immediate indication of who the owner of the digital work is, and data watermarked with visible watermarks are not of as much usefulness to a potential pirate (because the watermark is visible). Invisible watermarks, on the other hand, increase the likelihood of prosecution after the theft has occurred. These watermarks should therefore not be detectable to thieves, otherwise they would try to remove it; however, they should be easily detectable by the owners.
A further classification of watermarks is into fragile, semi-fragile or robust:
a. A fragile watermark is embedded in digital data to for the purpose of detecting any changes that have been made to the content of the data. They achieve this because they are distorted, or "broken", easily. Fragile watermarks are applicable in image authentication systems.
b. Semi-fragile watermarks detect any changes above a user-specified threshold.
c. Robust watermarks are designed to survive "moderate to severe signal processing attacks".
Watermarks for images can further be classified into spatial or spectrum watermarks, depending on how they are constructed:
a. Spatial watermarks are created in the spatial domain of the image, and are embedded directly into the pixels of the image. These usually produce images of high quality, but are not robust to the common image alterations.
b. Spectral (or transform-based) watermarks are incorporated into the image's transform coefficients. The inverse-transformed coefficients form the watermarked data.
Perceptual watermarks are invisible watermarks constructed from techniques that use models of the human visual system to adapt the strength of the watermark to the image content. The most effective of these watermarks are known as image-adaptive watermarks.
Finally, blind watermarking techniques are techniques that are able to detect the watermark in a watermarked digital item without use of the original digital item.
3. EFFECTIVE DIGITAL WATERMARKS
3.1 Features of a Good Watermark
The following are features of a good watermark:
1. It should be difficult or impossible to remove a digital watermark without noticeably degrading the watermarked content. This is to ensure that the copyright information cannot be removed.
2. The watermark should be robust. This means that it should remain in the content after various types of manipulations, both intentional (known as attacks on the watermark) and unintentional (alterations that the digital data item would undergo regardless of whether it contains a watermark or not). These are described below. If the watermark is a fragile watermark, however, it should not remain in the digital data after attacks on it, but should be able to survive certain other alterations (as in the case of images, where it should be able to survive the common image alteration of cropping).
3. The watermark should be perceptually invisible, or transparent. That is, it should be imperceptible (if it is of the invisible type). Embedding the watermark signal in the digital data produces alterations, and these should not degrade the perceived quality of the data. Larger alterations are more robust, and are easier to detect with certainty, but result in greater degradation of the data.
4. It should be easy for the owner or a proper authority to readily detect the watermark. "Such decodability without requiring the original, unwatermarked [digital document or] image would be necessary for efficient recovery of property and subsequent prosecution".
Further properties that enhance the effectiveness of a watermarking technique, but which are not requirements are:
5. Hybrid watermarking refers to the embedding of a number of different watermarks in the same digital carrier signal. Hybrid watermarking allows intellectual property rights (IPR) protection, data authentication and data item tracing all in one go.
6. Watermark key: it is beneficial to have a key associated with each watermark that can be used in the production, embedding, and detection of the watermark. It should be a private key, because then if the algorithms to produce, embed and detect the watermark are publicly known, without the key, it is difficult to know what the watermark signal is. The key indicates the owner of the data.
It is of interest to identify the properties of a digital data item (the carrier signal) that assist in watermarking:
1. It should have a high level of redundancy. This is so that it can carry a more robust watermark without the watermark being noticed. (A more robust watermark usually requires a larger number of alterations to the carrier signal).
2. It must tolerate at least small, well-defined modifications without changing its semantics.
5. Attacks on Watermarks
· Lossy Compression: Many compression schemes like JPEG and MPEG can potentially degrade the data’s quality through irretrievable loss of data.
· Geometric Distortions: include such operations as rotation, translation, scaling and cropping.
· Common Signal Processing Operations: They include the followings.
· D/A conversion, A/D conversion
· Resampling, Requantization, Recompression
· Linear filtering such as high pass and low pass filtering.
· Addition of a constant offset to the pixel values
· Local exchange of pixels
· other intentional attacks:
· Printing and Rescanning
· Watermarking of watermarked image (rewatermarking)
6. DIGITAL WATERMARKING APPLICATIONS
Digital watermarking is rapid evolving field, this section identifies digital watermarking applications and provides an overview of digital watermarking capabilities and useful benefits to customers. The various applications are:
· Authentication
· Broadcast Monitoring
· Copy Prevention
· Forensic Tracking
· E-Commerce/Linking
AUTHENTICATION
Authentication identifies if content has been altered or falsified. For example digital watermarking can verify authenticity and identify counterfeiting as a second layer of security for encrypted content. The presence of digital watermark and/or continuity of watermark can help ensure that the content has not been altered.
BROADCAST MONITORING
Broadcast content is embedded wit a unique identifier, and optionally, distributor information. Detectors are placed at popular markets, where broadcasts are received and processed, resulting in reports to be sent to the owner.
COPY PREVENTION
Copy prevention helps the digital watermarks to identify whether the content can be copied. It guards against unauthorized duplication.
FORENSIC TRACKING
Forensic tracking locates the source of the content. The key advantage of digital watermarking is that it enables tracking of the content to where it leaves an authorized path.
E-COMMERCE/LINKING
The digital watermarking enables the user to purchase or access information about the content, related content, or items with in the content.
7. WATERMARKING SOFTWARE&SREVICES
1. Alpha-Tec: watermarking software for copyright protection and infringement tracking.
2. Digimarc: For document verification, copyright protection, embedded messages and more.
3. Stegnosign: For creating, embedding and detecting watermarks.
4. Signum: Allow digital fingerprints to be embedded into grahics, audio, video e.t.c.
5. MediaSec: Provide software for various media types, partial encryption, and internet tracking.
8. CONCLUSION
Digital watermarks have been used in the last few years to protect the ownership of digital data. Various techniques developed make use of the human audio-visual system. Legitimate business and webmasters have nothing to fear from copyright law or new form of on-line enforcement technology found in digital watermarks and tracking services. By using audio files and images only when they have obtained permission of the appropriate owner, webmasters should be free to continue making their sites audio visually appealing.
9. REFERENCES
1. Electronics For You
4. www.digitalwatermarking.com
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