Tracker trees for unusual event detection

We present an approach for unusual event detection, based on a tree of trackers. At lower levels, the trackers are trained on broad classes of targets. At higher levels, they aim at more specific targets. For instance, at the root, a general blob tracker could operate which may track any object. The next level could already use information about human appearance to better track people. A further level could go after specific types of actions like walking, running, or sitting. Yet another level up, several walking trackers can be tuned to the gait of a particular person each. Thus, at each layer, one or more families of more specific trackers are available. As long as the target behaves according to expectations, a member of a higher up such family will be better tuned to the data than its parent tracker at a lower level. Typically, a better informed tracker performs more robustly. But in cases where unusual events occur and the normal assumptions about the world no longer hold, they loose their reliability. In such cases, a less informed tracker, not relying on what has now become false information, has a good chance of performing better. Such performance inversion signals an unusual event. Inversions between levels higher up represent deviations that are semantically more subtle than inversions lower down: for instance an unknown intruder entering a house rather than seeing a non-human target.