New Technique Promises Better Digital Watermarks

When New York City recently started arming police officers with digital cameras to take pictures of the injuries suffered by domestic abuse victims, convictions soared. The digital photos were clearer than those produced by the Polaroid cameras the police were using before, and could be easily blown up for use in the courtroom.

But those digital images could be tampered with, claimed critics, unless they contained a digital watermark proving their authenticity. And until now, embedding a digital watermark has meant a loss of image quality when enlarging an image.

This week, however, researchers at the University of Rochester and Xerox Corp announced that they have discovered a way to embed information in digital images, without distorting the original document.

Called “reversible data hiding,” the new technique holds the promise of solving a dilemma faced by digital image users, particularly in sensitive military, legal and medical applications.

Current digital watermarking techniques irreversibly change the image, resulting in distortions or information loss. “While these distortions are often imperceptible or tolerable in normal applications,” says Gaurav Sharma, one of the Xerox scientists who developed the new approach, “if the image is enlarged, enhanced, or processed using a computer, the information loss can be unacceptable.”

“The greatest benefit of this technology is in determining if anyone has clandestinely altered an image,” explained Murat Tekalp, a University of Rochester electrical engineering professor who also worked on the research. “These days many commercial software systems can be used to manipulate digital images. By encoding data in this way we can be sure the image has not been tampered with, and then remove the data within it without harming the quality of the picture.”

While it will probably take some time before the technology is translated into commercial products, it likely be useful not just for law enforcement, but for a broad range of commercial applications, according to Matt Jackson, a professor of communications at Pennsylvania State University.

“Digital watermarking is one of the key components to making content available online,” Jackson says. “By providing a way to track down and prove copyright infringements, it gives people confidence that they can control the distribution of their product.”

In addition, recent legislation, such as the 1998 Digital Millennium Copyright Act (DCMA), as well as international copyright treaties, are beginning to require that anyone distributing a copyrighted work keep the copyright owner’s information with the work. “Digital watermarking is one of the best ways to keep that information with the work,” says Jackson. “By facilitating that, this technology could help make more content available online.”