Digital watermarking tool no panacea for Web images – DigiMarc

Digital watermarking tool no panacea for Web images – DigiMarc – Company Business and Marketing

Bruce Fraser

In the CyberTimes section of the Nov. 11 issue of The New York Times, Marty Katz pointed out that many Web images – despite the efforts of their creators to digitally protect them with DigiMarc watermarking software – don’t have readable marks. To those of us who have worked extensively with the DigiMarc technology, this came as no surprise, though it may be a shock to the content creators. It’s not that the technology doesn’t work, but it’s more limited than either DigiMarc Corp. or its major adopters such as Adobe Systems Inc. and Corel Corp. would have us believe.

Pixel logic

DigiMarc’s watermarking works by making small changes to the luminance of individual pixels, creating a pattern in the image that, if all goes well, remains below the threshold of visibility. The DigiMarc plug-in bundled with Adobe Photoshop offers watermarks in four strengths; the trade-off is between keeping the watermark invisible and making it sufficiently robust to be detectable even after the image has been manipulated. The two higher settings produce watermarks that are visible in most images, so many users choose the lower two settings to maintain image fidelity. Watermarks on these lower settings are quite fragile.

The third major variable is image size, which seems to be causing most of the problems with Web images. DigiMarc recommends watermarking images that are at least 256 by 256 pixels, with a minimum of 100 by 100 pixels. An afternoon’s informal testing suggests that this recommendation is on the optimistic side, particularly when the effects of JPEG compression are also factored in.

Muddy waters

The smaller the image, the fewer pixels the watermarking algorithm has to work with. It’s difficult to watermark a small image without visibly degrading it. At best, there’s some loss of highlight contrast; at worst, an obvious pattern appears. With the 1.0 version of the plug-ins that ship with Photoshop, we found that simply saving a 600-by-400-pixel image as a JPEG was often enough to wipe out a watermark embedded at strengths 1 or 2.

DigiMarc is continuing to work on the problem. A new set of plug-ins, Version 1.51, is available to download free from http://www.digimarc.com, and the robustness of the watermarking is considerably improved, particularly for small JPEGs. However, the older plug-ins cannot always read the watermarks generated by the new version, and obviously the upgrade does nothing to address the large number of existing images with insufficient watermarks.

Perhaps the most troubling aspect of all this is that users who make good-faith efforts to use the technology may assume that if an image lacks a readable watermark, it isn’t copyrighted. That is simply not the case – the only safe assumption is that all images are copyrighted. Digital watermarking should be seen as a tool to help track down the copyright holder, nothing more.

Bruce Fraser welcomes correspondence at bruce@pixelboyz.com.

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