HMM-based human motion recognition with optical flow data

Human motion recognition is traditionally approached by either recognizing basic motions from features derived from video input or by interpreting complex motions by applying a high-level hierarchy of motion primitives. The former method is usually limited to rather simple motions while the latter requires human expert knowledge to build up a suitable hierarchy. In this paper we propose a new approach that uses the strength of both methods while overcoming their respective limitations. Our approach is able to recognize the motion units within complex motion sequences. The recognition process applies hidden Markov models (HMM) based on features consisting of optical flow gradient histograms. For each primitive motion unit we train one HMM and then concatenate these primitive motion units to form complex motion sequences. Modeling sequences with HMMs allows for a very flexible combination of motion units into motion sequences. They can either be combined in a restrictive rule-based formulation using predefined grammars or be more flexibly combined using a statistical model of sequence probabilities. In this paper we are mainly interested in the comparison of the optical flow features with marker-based features, therefore we do not use a motion grammar. We apply our approach to 24 motion units forming five complex motion sequences as they appear in a real-world kitchen tasks. The results show that the proposed approach allows for a very fast low-level recognition of human motion units without the need for any complex reconstruction, post processing or pose estimation. Straight-forward characteristic flow fields in combination with HMM sequence modeling are sufficient to reliably recognize complex motions even with an unrestricted search. Our results show that this search already achieves 13.1 % recognition error rate. We compare HMM models based on the optical flow features to those derived from a marker-based system. Our recognition results indicate that optical flow features achieve a competitive performance.