Sometimes, a photo is simply too good to be true.
Tiny details in an image, for instance, may be too similar to have occurred naturally, suggesting a cut-and-paste maneuver. Or the color patterns may be too "normal" _ beyond the limitations of sensors on digital cameras.
A growing number of researchers and companies are looking for such signs of tampering in hopes of restoring credibility to photographs at a time when the name of a popular program for manipulating digital images has become a verb, Photoshopping.
Adobe Systems Inc., the developer of Photoshop, said it may incorporate their techniques into future releases.
"There's much more awareness and much more skepticism when (people) are looking at images," said Kevin Connor, a senior director of product management at Adobe. "That's why we think that's something we need to get involved in. It's not healthy to have people be too skeptical about what they saw."
Meanwhile, camera maker Canon Inc. sells a data-verification kit with some models. It can stamp digital photos with an invisible, mathematical summary of the image, such that even one tiny change will produce a mismatch and flag the photo as an alteration.
These techniques are of interest to law-enforcement officials and defense attorneys because photographic evidence can make or break cases. News organizations also have been increasingly exploring ways to spot hoaxes.
Not everyone is on board, however.
Michael Cherry, vice chairman of the Digital Technology Committee at the National Association of Criminal Defense Lawyers, said too little is known about the nuances of digital photography to put much trust in such detection techniques.
For example, Cherry said, using a photo printer rather than a laser printer can make a color image look nicer but lose details, such that a gun appears slightly off. The detection tools would reveal nothing, he said, because the photo itself was never digitally tweaked.
Meanwhile, enhancements to bring out details, such as sharpening a fuzzy surveillance image, may inadvertently turn a dark spot into something that looks like a gun.
And when there is intent to deceive, people who have enough money, time and skills can cover their tracks and evade any tamper detection, he said.
Lawyers and juries ultimately have to consider circumstances beyond the image itself, Cherry said. For example, was the evidence available at the time of the dispute or did it suddenly show up six months later _ giving that person time to manufacture it?
Researchers stand by their techniques.
"There will always be a countermeasure that cannot be prevented," said Jessica Fridrich, a professor at Binghamton University. "We are trying to make it harder for people who want to do these things to go unnoticed, undetected."
The key, she said, is to use tools in combination. A criminal or hoaxer might be sophisticated enough to defeat one technique, but not all at once.
Fridrich's research takes advantage of the fact that all cameras have tiny flaws, so small they don't affect what the eye can see. For example, her software could analyze a set of photographs taken by the same camera and notice that a certain, defective pixel is always dark. Seeing that pixel light up would suggest an alteration.
Dartmouth College professor Hany Farid, meanwhile, has developed a set of software tools he collectively calls Q-IF. He sells the programs for up to $25,000 (euro16,872) a year.
One tool looks for the use of clone stamp, a feature for duplicating or erasing objects in an image. Two cloned flowers would appear identical and lack expected blemishes.
Another exploits how cameras capture color images. Color is a mixture of red, green and blue. Rather than have sensors that detect all three for each pixel, they generally alternate in a specific pattern. That pattern gets disrupted with airbrushing.
Other techniques include looking for inconsistencies in lighting and shadows.
A human still must make a final determination, and Farid admits he can never be certain. His techniques got challenged in one criminal case, and prosecutors withdrew him as an expert witness.
"If we don't find traces of tampering, we don't say it's real," Farid said. "We say we find no traces of tampering. That's the best we can say."
Nonetheless, his tools are innovative enough to pique the interest of Adobe, which is subsidizing his research.
Photoshop already has a logging feature, which can track and record every change made along the way _ standard procedure these days in law enforcement.
"You have an audit trail, even if you have gone in and made changes to the image," said Cynthia Baron, author of "Adobe Photoshop Forensics: Sleuths, Truths and Fauxtography."
Adobe has no specific release schedule, though, on tamper-detection tools. The worry is that these same tools can help hoaxers test whether their changes escape notice.
"One of the things we've got to tackle," Conner said, "is to figure out if we can put some of these features in without making it easier for people to thwart them."
Sometimes, a photo is simply too good to be true.