Detecting Photoshop Fakeries

Prof. Hany Farid at Dartmouth is the leading researcher in developing mathematical [and just plain common sense] analytical techniques for detecting Photoshop manipulation. As we know, with Photoshop one can work at the pixel level to change images, and on the pixel level there is no way to detect changes. That's been the starting point in “digital truth” discussions. But it also makes sense to think that many forms of manipulation leave traces in the over logic of pixels in an image: messing with the real almost always leaves some disturbance in in the force. As Science News reports:

“We've developed a bag of tricks,” Farid says. “Every time somebody tampers with a photograph, we try to understand what they did and how to detect it.'

In tampering with the Reuters photograph the forger used ”copying and pasting,“ a common forgery technique. A forger may also use copying and pasting to remove a person's image from a photograph by covering it. In either case, Farid's software can identify the forgery by detecting repetitions in the digital bits that comprise the image—even if those repetitions are too subtle for the eye to detect.

Another way to doctor an image is to piece together two separate photographs. For example, during the 2004 presidential campaign, an image surfaced on the Web showing John Kerry speaking with Jane Fonda at an anti-war demonstration in the 1960s, complete with an Associated Press insignia. [...] In this case, the original photographers recognized that their own work had been altered, thus exposing the forgery. However, Farid could also have detected the forgery by analyzing the pixels. In the originals, Kerry and Fonda were different sizes, so the forger had to rescale Fonda to put her into the photograph with Kerry. That rescaling leaves a telltale signature in the pixels.

Farid also analyzes the light in photographs to see if the light source is the same across different portions of the image. One way of doing that is to study the shadows to extrapolate where the light must have been coming from. Another way is to study the dots in people's pupils.

”The eyes are a partial mirror into the world in which you're photographed,“ Farid says. If there are two white dots in each eye, there had to have been two separate light sources. So, if a photo shows two dots in one person's eyes and only one dot in another person's eyes, it must have been spliced together from two different originals. From the left, the first and third persons have a single dot in each pupil, while the second and fourth have two dots in each eye. Therefore, the image must be a composite.

Also, the color of the light determines the dots' precise shade of white. A composite image from different photographs may have shades that vary from person to person.
Each latest version of Photoshop has new tools that allow for better forgeries, so Farid continually needs to figure out new methods

”This is an arms race,“ Farid says. ”I can already tell you how it's going to end: We're going to lose. It's always going to be easier to create a forgery than detect a forgery. But we're going to take the power to create forgeries out of the hands of amateurs. We will raise that bar up until you have to be very, very good to do it.“

Comments

[Richard Lynch]

Detecting fakes can also often be accomplished by making layered comparisons with threshold, layer modes, etc. Examining channel information or making specialized separations can also lead to clues. There was one photo I remember seeing in the news (in fact National Geographic: http://news.nationalgeographic.com/news/2006/07/060720-lobster-photo.html) about a lobster that was half blue and half red some time last year. It is said to be 'very rare' ... it seems to me that type of thing would be easy to do as a color-adjustment forgery that would be virtually undetectable. All you need is a good selection and the right technique.