1. A method of tracking an object in a video stream comprising a plurality of video frames, the method comprising: (a) running an object detector at a plurality of sampling locations, the locations defining a first grid spaced across a first frame, and recording a hit at each location where an object of interest is found; and (b) running the object detector at a further plurality of sampling locations defining a second grid spaced across a second frame, the second grid being offset from the first grid, and running the detector in addition at one or more further locations on the second frame derived from the or each location on the first frame at which a hit was recorded.