Exposing Photo Manipulation from Shading and Shadows
We describe a method for detecting physical inconsistencies in lighting from the shading and shadows in an image. This method imposes a multitude of shading- and shadow-based constraints on the projected location of a distant point light source. The consistency of a collection of such constraints is posed as a linear programming problem. A feasible solution indicates that the combination of shading and shadows is physically consistent, while a failure to find a solution provides evidence of photo tampering.
Eric Kee, James F. O'Brien, and Hany Farid. "Exposing Photo Manipulation from Shading and Shadows". ACM Transactions on Graphics, 33(5):165:1–165:21, September 2014. Presented at SIGGRAPH 2014.