I was recently asked by a media organization to analyze and comment on the authenticity of President Obama’s birth certificate — some stories just don’t seem to every go away. Over the years I’ve been asked many times to perform such an analysis and/or comment on the purported evidence of fakery. I have repeatedly demurred because it has been my experience that facts have no place in a good conspiracy.
Welcome to the Fourandsix blog, where you’ll find tips on image forensics techniques and commentary on issues relevant to photo tampering and the responsible use of imaging tools.
My last blog post touched on the use of Photoshop by the North Korean and Iranian governments to puff up their military might. This week, I’d like to contrast the digital manipulation practiced by those governments with China, a country that seems to have a differerent kind of Photoshop fixation. Whereas North Korea and Iran often enlist Photoshop for a kind of photographic chest thumping, the most notable examples of photo-manipulation coming from China seem to revolve around putting local government officials in places or positions that they never actually went.
Over the past few years, it almost seems as if Iran and North Korea are in a virtual arms race. The competition centers on who can produce the more outrageously doctored photo showcasing their supposed military might. Of course, the most notorious example of this is the Iranian missile photo that makes multiple appearances in our Photo Tampering Gallery. The most recent example, however, comes from North Korea. As was revealed by Alan Taylor at The Atlantic last week, a photo released by the Korean Central News Agency that purports to show a squadron of hovercraft involved in a military exercise appears to have been manipulated to increase the number of hovercraft. As with the Iranian missile photo, this incident illustrates how easy it can be to identify sloppily cloned image elements when armed with a sharp eye and a copy of Photoshop. But what about work that’s not quite so sloppy?
Most digital cameras capture color images using a single sensor and an array of color filters. As a result, only one-third of the samples in a color image are captured by the camera. The other two-thirds are computed by the camera software — a process known as color filter array (CFA) interpolation or demosaicking. This interpolation introduces specific patterns of correlation in a digital image. Disruptions of these patterns can therefore be used to detect and localize tampering in an image.
Over a hundred years ago, Simon Newcomb observed a surprising pattern in the distribution of the leading digits in logarithm tables: the digit 1 is significantly more likely to occur than the digit 2, which is more likely to occur than the digit 3, and so on. More than fifty years later, Frank Benford rediscovered this same pattern in more data sets such as the stock market, census data, accounting data, and more. Because fabricated data tends not to follow this same pattern, this phenomenological law has been used to detect accounting, tax, and scientific fraud. More recently, this law has been applied to detect various forms of image tampering.