Majeure "Mathématiques et Informatique" Enseignement d'approfondissement "Images: Analyse et Synthèse"

Introduction

The enormous popularity of the World Wide Web in the early 1990's demonstrated the commercial potential of offering multimedia resources through the digital networks. Since commercial interests seek to use the digital networks to offer digital media for profit, they have a strong interest in protecting their ownership rights. Digital watermarking has been proposed as one way to accomplish this.

A digital watermark is a digital signal or pattern inserted into a digital image. Since this signal or pattern is present in each unaltered copy of the original image, the digital watermark may also serve as a digital signature for the copies. A given watermark may be unique to each copy (e.g. to identify the intended recipient), or be common to multiple copies (e.g. to identify the document source). In either case, the watermarking of the document involves the transformation of the original into another form. This distinguishes digital watermarking from digital fingerprinting, where the original file remains intact and a new created file 'describes' the original file's content.

Digital watermarking is also to be contrasted with public-key encryption, which also transform original files into another form. It is a common practice nowadays to encrypt digital documents so that they become un-viewable without the decryption key. Unlike encryption, however, digital watermarking leaves the original image (or file) basically intact and recognizable. In addition, digital watermarks, as signatures, may not be valitdated without special software. Further, decrypted documents are free of any residual effects of encryption, whereas digital watermarks are designed to be persistent in viewing, printing, or subsequent re-transmission or dissemination.

The purpose of digital watermarks

Two types of digital watermarks may be distinguished, depending upon whether the watermark appears visible or invisible to the casual viewer.
"Visible watermarks" are used in much the same way as their bond paper ancestors, where the opacity of paper is altered by physically stamping it with an identifying pattern.
This is done to mark the paper manufacturer or paper type. One might view digitally watermarked documents and images as digitally 'stamped'.

Invisible watermarks, on the other hand, are potentially usefull as a means of identifying the source, author, creator, owner, distributor or authorized consumer of a document or image. For this purpose, the objective is to permanently and unalterably mark the image so that the credit or assignment is beyond dispute. In the event of illicit usage, the watermark would facilitate the claim of ownership, the receipt of copyright revenues, or the success of prosecution.

Watermarking has also been proposed to trace images in the event of their illicit redistribution. Whereas past infringement with copyrighted documents was often limited by the unfeasibility of large-scale photocopying and distribution, modern digital networks make large-scale disseminaton simple and inexpensive. Digital watermarking makes it possible to uniquely mark each image for every buyer. If that buyer then makes an illicit copy, the illicit duplication may be convincingly demonstrated.

Visible vs. invisible watermarks

Visible and invisible watermarks both serve to deter theft but they do so in very different ways.
"Visible watermarks" are especially useful for conveying an immediate claim of ownership. The main advantage is that they virtually eliminate the commercial value of the document to a would-be thief without lessening the document's utility for legitimate, authorized purposes. A familiar example is in the video domain where a logo is placed in a corner of the screen image.
Invisible watermarks, on the other hand, are more of an aid in catching the thief than discouraging the theft in the first place.
In general, "visible watermarks" diminish the commercial value of a document or image, whereas invisible watermarks increase the likelihood of successful prosecution. The invisible watermark may also act as a deterrent if perpetrator is aware of their possible use.

Requirements of watermarks

To be effective in the protection of the ownership of intellectual property, the invisibly watermarked image should satisfy several criteria:

1. the watermark must be difficult or impossible to remove, at least without visibly
    degrading the original image,
2. the watermark must survive image modifications that are common to typical
    image-processing applications (e.g. scaling, dithering, cropping, compression),
3. an invisible watermark should be imperceptible so as not to affect the experience of
    viewing the image, and
4. for some invisible watermarking applications, watermarks should be readily detectable by
    the proper authorities, even if imperceptible to the average observer. Such decodability
    without requiring the original, un-watermarked image would be necesary for efficient
    recovery of property and subsequent prosecution.

Techniques for watermarking

Several different methods enable watermarking in the spatial domain. The simplest (too simple for many applications) is just to flip the lowest-order bit of chosen pixels. This works well only if the image is not subject to any modification.
A more robust watermark can be embedded by superimposing a symbol over an area of the picture. The resulting mark may be visible or not, depending upon the intensity value. Picture cropping, e.g., (a common operation of image editors), can be used to eliminate the watermark.
Spatial watermarking can also be applied using color separation. In this way, the watermark appears in only one of the color bands. This renders the watermark visibly subtle such that it is difficult to detect under regular viewing. However, the mark appears immediately when the colors are seperated for printing. This renders the document useless for the printer unless the watermark can be removed from the color band. This approach is used commercially for journalists to inspect digital pictures from a photo-stockhouse before buying unmarked versions.
Watermarking can be applied in the frequency domain (and other transform domains) by first applying a transform like the Fast Fourier Transform (FFT). In a similar manner to spatial domain watermarking, the values of chosen frequencies can be altered from the original. Since high frequencies will be lost by compression or scaling, the watermark signal is applied to lower frequencies, or better yet, applied adaptively to frequencies that contain important information of the original picture. Since watermarks applied to the frequency domain will be dispersed over the entirety of the spatial image upon inverse transformation, this method is not as susceptible to defeat by cropping as the spatial technique. However, there is more a tradeoff here between invisibility and decodability, since the watermark is in effect applied indiscriminately across the spatial image.

Limitations of digital watermarking

In january 1997, a counterfeiting schemes has been demonstrated for a class of invertible, feature-based, frequency domain, invisible watermarking algorithms. This counterfeiting scheme could be used to subvert ownership claims because the recovery of the digital signature from a watermarked image requires a comparison with an original. The counterfeiting scheme works by first creating a counterfeit watermarked copy from the genuine watermarked copy by effectively inverting the genuine watermark. This inversion creates a counterfeit of the original image. A comparison of the decoded versions of both yield the owner's (authorized) and the forged (inverted) signature. This, the technique of establishing legitimate ownership recovering the signature watermark by comparing a watermarked image with the original image breaks down. It can be shown that both the legitimate signature and counterfeiter's signature inhere in both the watermarked and counterfeit watermarked copies. Thus, while it may be demonstrated that at least one recipient has a counterfeit watermarked copy, it can not be determined which it is.

This research suggests that not all watermarking techniques will be useful in resolving ownership disputes in courts of law. There will likely be non-commercial applications, or those with limited vulnerability to theft, where "good enough watermarking" will suffice. More sensitive applications may require non-invertable or non-extracting watermarking techniques. These issues are under consideration right now.