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Digital detectives discern Photoshop fakery

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Over the past six years, computer science professor Hany Farid has become something of a digital detective. While Paris Match's virtual liposuction was exposed because the unaltered photo ran in several other publications that week (including the Aug. 6 Monitor), Mr. Farid doesn't need the original to reveal tampering.

As head of Dartmouth College's Image Science Group in Hanover, N.H., he's developed computer algorithms that can tease out the tiny flaws hidden in phony photos.

"There's no way to push a button and tell if it's real, but there are tests we can run that allow us to be pretty sure if it's a fake," says Farid.

Some of the investigative techniques are simply teaching a computer to spot what the untrained eye is too lazy to see. If a figure from one photo has been edited into another, there are almost always imperfections – subtle inconsistencies in the physics and geometry of the combined image. The vanishing points might be off, or the shadows cast from two or more objects may contradict one another.

"These are things humans are really bad at noticing," says Farid. But to a computer, the subtle differences are obvious.

Farid can now run possible forgeries through a gamut of tests, even checking the light reflections in people's eyes to triangulate the location of the flash camera that took the picture. If the analysis of subjects in a photo shows that the camera had to be in multiple places at once, the shot's a fake.

Courts face off with digital fraudsters

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