1. A method in a computing device for identifying a root correspondence for previously detected objects and newly detected objects, the method comprising: for pairs of a previously detected object and a newly detected object, evaluating the objects of the pair as having a root correspondence by calculating a mapping efficiency indicating how well the newly detected object corresponds to the previously detected object based on locations of the other newly detected objects relative to locations of other previously detected objects wherein the mapping efficiency is a fraction of newly detected objects that are proximate to a previously detected object when spatially shifted by an offset from the newly detected object of the pair to the previously detected object of the pair; and selecting a pair of a previously detected object and a newly detected object as having a root correspondence based on the calculated mapping efficiency of that pair.