The United States military has become increasingly reliant on digital images from drones and satellites to give soldiers a sense of the battlefield. Law enforcement officers routinely use digital cameras to photograph crime scenes. Newspapers and magazines are now dependent on digital photographs that can be easily doctored.
Over the last three years, Professor Farid and his students have become experts at forgery, making hundreds of images that look authentic but have in fact been digitally tweaked. License plate numbers are changed. A single stool standing on a checkerboard floor is suddenly a pair of stools. Dents on a car are wiped away with a few mouse clicks.
The skillful tampering disturbed the images in ways that the human eye could not detect. But Professor Farid says his algorithms can spot them and sound the alarm.
For example, when two images are spliced together – like the picture of a shark attacking a helicopter that has circulated around the Internet in the past few years – one or both of the original pictures usually has to be shrunk, enlarged or rotated to make the pieces fit together. And those changes, no matter how artful, leave clues behind.
Take a picture that is 10 pixels by 10 pixels, for a total of 100. Stretch it to 10 by 20 pixels, and image-editing software like Adobe Photoshop will assign the picture’s original pixels to every other slot in the new picture. That leaves 100 pixels “blank,” or without values. Image-editing software fills in the gaps by examining what their neighbors look like, and then applying an average. To oversimplify, if pixel A is blue, and pixel C is red, the blank pixel B will become purple.