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The Paparazzi-Detector Test

By Elizabeth SvobodaJanuary 21, 2006 6:00 AM


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Did tabloid photographers really snap embarrassing pictures of a topless Britney Spears? Or were the shots digital manipulation? Engineer Lorenzo Cozzella of the Third University of Rome in Italy has devised a way to detect digital forgery, allowing investigators to determine whether an electronic image is legitimate.

Innovative as the new antifraud technique is, it's inspired by a method that has been around for centuries: watermarking. Cozzella and his colleagues developed a foolproof variation that involves removing random pixels that don't contribute to the image and inserting an invisible digital watermark, such as a corporate logo, in their place. The watermark is encrypted so that only someone with a private code can verify its presence.

Any modification to the photo—including airbrushing or tinting—will cause damage that the tamperer won't see. Because the encrypted watermark is a computer-generated hologram, it can be easily reconstructed in its entirety no matter how damaged it is. As a result, investigators can ascertain a photo's authenticity using only a few pixels of visual information. "You could use this technique to make sure that military satellite images are original," he says, "or that a medical image presented during a trial is authentic." 

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