Backtracking: Retrospective multi-target tracking

摘要

We introduce a multi-target tracking algorithm that operates on prerecorded video as typically found in post-incident surveillance camera investigation. Apart from being robust to visual challenges such as occlusion and variation in camera view, our algorithm is also robust to temporal challenges, in particular unknown variation in frame rate. The complication with variation in frame rate is that it invalidates motion estimation. As such, tracking algorithms based on motion models will show decreased performance. On the other hand, appearance based detection in individual frames suffers from a plethora of false detections. Our tracking algorithm, albeit relying on appearance based detection, deals robustly with the caveats of both approaches. The solution rests on the fact that for prerecorded video we can make fully informed choices; not only based on preceding, but also based on following frames. We start off from an appearance based object detection algorithm able to detect in each frame all target objects. From this we build a graph structure. The detections form the graph’s nodes and the vertices are formed by connecting each detection in a frame to all detections in the following frame. Thus, each path through the graph shows some particular selection of successive detections. Tracking is then reformulated as a heuristic search for optimal paths, where optimal means to find all detections belonging to a single object and excluding any other detection. We show that this approach, without an explicit motion model, is robust to both the visual and temporal challenges.