SeMoLi: What Moves Together Belongs Together
Authors: Jenny Seidenschwarz, Aljosa Osep, Francesco Ferroni, Simon Lucey, Laura Leal-Taixe

We tackle semi-supervised object detection based on motion cues. Recent results suggest that heuristic-based clustering methods in conjunction with object trackers can be used to pseudo-label instances of moving objects and use these as supervisory signals to train 3D object detectors in Lidar data without manual supervision. We re-think this approach and suggest that both object detection as well as motion-inspired pseudo-labeling can be tackled in a data-driven manner. We leverage recent advances in scene flow estimation to obtain point trajectories from which we extract long-term class-agnostic motion patterns. Revisiting correlation clustering in the context of message passing networks we learn to group those motion patterns to cluster points to object instances. By estimating the full extent of the objects we obtain per-scan 3D bounding boxes that we use to supervise a Lidar object detection network. Our method not only outperforms prior heuristic-based approaches (57.5 AP +14 improvement over prior work) more importantly we show we can pseudo-label and train object detectors across datasets.