How Good is Good Enough?: The Impact of Errors in Single Person Action Classification on the Modeling of Group Interactions in Volleyball

In Human Behaviour Understanding, social interaction is often modeled on the basis of lower level action recognition. The accuracy of this recognition has an impact on the system's capability to detect the higher level social events, and thus on the usefulness of the resulting system. We model team interactions in volleyball and investigate, through simulation of typical error patterns, how one can consider the required quality (in accuracy and in allowable types of errors) of the underlying action recognition for automated volleyball monitoring. Our proposed approach simulates different patterns of errors, grounded in related work in volleyball action recognition, on top of a manually annotated ground truth to model their different impact on the interaction recognition. Our results show that this can provide a means to quantify the effect of different type of classification errors on the overall quality of the system. Our chosen volleyball use case, in the rising field of sports monitoring, also addresses specific team related challenges in such a system and how these can be visualized to grasp the interdependencies. In our use case the first layer of our system classifies actions of individual players and the second layer recognizes multiplayer exercises and complexes (i.e. sequences in rallies) to enhance training. The experiments performed for this study investigated how errors at the action recognition layer propagate and cause errors at the complexes layer. We discuss the strengths and weaknesses of the layered system to model volleyball rallies. We also give indications regarding what kind of errors are causing more problems and what choices can follow from them. In our given context we suggest that for recognition of non-Freeball actions (e.g. smash, block) it is more important to achieve a higher accuracy, which can be done at the cost of accuracy of classification of Freeball actions (which are mostly plays between team members and are more interchangable as to their role in the complexes).