To test how well it worked, Karsch et al. showed some study participants a series of images — some with no synthetic objects, and some with synthetic objects inserted in one of three ways: either an existing light-derivation method, their new algorithm with a simplified lighting model, and their new algorithm in all its light-modeling glory. The subjects had computer science or graphics backgrounds.

“Surprisingly, subjects tended to do a worse job identifying the real picture as the study progressed,” the authors explain in a paper describing their method. “These results indicate that people are not good at differentiating real from synthetic photographs, and that our method is state of the art.”