Lou Franco's ECM Imaging Blog

A Chinese newspaper got caught altering a photograph:

It all started in 2006 when China neared completion of its massive, high-altitude, $4 billion Qinghai-Xizang railway that connects Tibet with "China proper." Environmentalists had protested loudly against the railway as its launch drew closer because it threatened the native habitat of an endangered antelope in China called the chiru. Coincidentally, it was about that time when a photograph appeared in Chinese newspapers by Liu Weiquing, a photographer with the Xinhua news agency.

It appears that this was figured out by a blogger who looked at an enlarged version and questioned specific artifacts. If you read my last post on JPEG encoding, you might appreciate this technique for finding doctored photographs.

Using a program he wrote (and provided on the conference CD-ROM) Krawetz could print out the quantization tables in a JPEG file (that indicate how the image was compressed) and determine the last tool that created the image -- that is, the make and model of the camera if the image is original or the version of Photoshop that was used to alter and re-save the image.

Comparing that data to the metadata embedded in the image he could determine if the photo was original or had been re-saved or altered. Then, using error level analysis of an image he could determine what were the last parts of an image that were added or modified.

Error level analysis involves re-saving an image at a known error rate (90%, for example), then subtracting the re-saved image from the original image to see every pixel that changed and the degree to which it changed. The modified versions will indicate a different error level than the original image.