  From an image sequence captured by a stationary camera, background
subtraction can detect moving foreground objects in the scene. Distinguishing
foreground from background is further improved by various heuristics. Then each
object's motion can be emphasized by duplicating its positions as a motion
trail. These trails clarify the objects' spatial relationships. Also, adding
motion trails to a video before previewing it at high speed reduces the risk of
overlooking transient events.
