Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for training an event detector. The methods, systems, and apparatus include actions of identifying a portion of a first interframe difference image that represents motion of an OI, determining that a second interframe difference image represents motion by a non-OI, combining the portion of the first interframe difference image and the second interframe difference image as a third interframe difference image labeled as motion of both an OI and a non-OI, and training an event detector with the third interframe difference image.