1. A method of determining costs between blob trackers and blobs, the method comprising: detecting a blob in a video frame, wherein the blob includes pixels of at least a portion of a foreground object; determining a physical distance between a blob tracker and the blob; determining a size ratio between the blob tracker and the blob; and determining a cost between the blob tracker and the blob using the physical distance and the size ratio, wherein the cost between the blob tracker and the blob is used to associate one or more of the blob trackers with one or more of the blobs.