Towards defining groups and crowds in video using the atomic group actions dataset

Understanding group activities is an essential step towards studying complex crowd behaviours in video. However, such research is often hampered by the lack of a formal definition of a group, as well as a dearth of datasets that concentrate specifically on Atomic Group Actions. ¹ In this paper, we provide a quantitative definition of a group based on the Group Transition Ratio (G_tr); the G_tr helps determine when individuals transition to becoming a group (where the individuals can still be tracked) or a crowd (where tracking of individuals is lost). In addition, we introduce the Atomic Group Actions Dataset, a set of 200 videos that concentrate on the atomic group actions of objects in video, namely the group-group actions of formation, dispersal, and movement of a group, as well as the group-person actions of person joining and person leaving a group. We further incorporate a structured, end-to-end analysis methodology, based on workflows, to easily and automatically allow for standardized testing of new group action models against this dataset. We demonstrate the efficacy of the G_tr on the Atomic Group Actions Dataset and make the full dataset (the videos, along with their associated tracks and ground truth, and the exported workflows) publicly available to the research community for free use and extension at at http://research. sethi.org/ricky/datasets/.