Exposing Photo Manipulation with Inconsistent Shadows
We describe a geometric technique to detect physically inconsistent arrangements of shadows in an image. This technique combines multiple constraints from cast and attached shadows to constrain the projected location of a point light source. The consistency of the shadows is posed as a linear programming problem. A feasible solution indicates that the collection of shadows is physically plausible, while a failure to find a solution provides evidence of photo tampering.
Eric Kee, James F. O'Brien, and Hany Farid. "Exposing Photo Manipulation with Inconsistent Shadows". ACM Transactions on Graphics, 32(4):28:1–12, September 2013. Presented at SIGGRAPH 2013.