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Two-Stream LSTM for Action Recognition with RGB-D-Based Hand-Crafted Features and Feature Combination

Good action recognition relies on correct interpretation of two critical attributes related to action: the spatial attribute on the detected person's posture, and the temporal attribute on the detected person's body movement. Whereas deep learning has greatly improved image recognition, we have not found a similar progress for action recognition. One of the main reasons is due to the complexity caused by the additional temporal dimension; another, to the fact that there are less annotated training data samples for action recognition than that for image recognition. In this regard, this paper proposes a handcrafted cued LSTM model for human action recognition based on RGB-D data, as a collection of 25 skeleton joints in 3D coordinates, found in NTU-RGB-D, currently the most comprehensive dataset for action recognition. As opposed to the raw data of skeleton joints, handcrafted cues, pre-processed results geared to facilitate focused learning, are proposed as input to the LSTM structure. In particular, pertaining to the spatial cue, the SVIT cue derived by Skeleton View-invariant Transformation is adopted; pertaining to the temporal cue, the Diff cue computed by taking the displacements of all joint across down-sampled raw data is utilized. Based on the train/test protocol, the experiment we conducted on NTU-RGB-D shows that the recognition result based on either of the proposed handcrafted cues is better than that based on the raw data. In addition, by our proposed techniques of feature fusion and/or decision fusion of these two handcrafted cues, the recognition performance is better than that of the state-of-the-art approaches conducting on the same dataset by same train/test protocol.