One key challenging issues of facial expression recognition (FER) in video sequences is to extract discriminative spatiotemporal video features from facial expression images in video sequences. In this paper, we propose a new method of FER in video sequences via a hybrid deep learning model. The proposed method first employs two individual deep convolutional neural networks (CNNs), including a spatial CNN processing static facial images and a temporal CN network processing optical flow images, to separately learn high-level spatial and temporal features on the divided video segments. These two CNNs are fine-tuned on target video facial expression datasets from a pre-trained CNN model. Then, the obtained segment-level spatial and temporal features are integrated into a deep fusion network built with a deep belief network (DBN) model. This deep fusion network is used to jointly learn discriminative spatiotemporal features. Finally, an average pooling is performed on the learned DBN segment-level features in a video sequence, to produce a fixed-length global video feature representation. Based on the global video feature representations, a linear support vector machine (SVM) is employed for facial expression classification tasks. The extensive experiments on three public video-based facial expression datasets, i.e., BAUM-1s, RML, and MMI, show the effectiveness of our proposed method, outperforming the state-of-the-arts. INDEX TERMS Facial expression recognition, spatio-temporal features, hybrid deep learning, deep convolutional neural networks, deep belief network.

Video-based facial expression recognition is a long-standing problem owing to a gap between visual features and emotions, difficulties in tracking the subtle movement of muscles and limited datasets.The key to solving this problem is to exploit effective features characterizing facial expression to perform facial expression recognition. We propose an effective framework to solve these problems. In our work, both spatial information and temporal information are utilized through the aggregation layer of a framework that fuses two state-of-the-art stream networks. We investigate different strategies for pooling across spatial information and temporal information. We find that it is effective to pool jointly across spatial information and temporal information for video-based facial expression recognition. Our framework is end-to-end trainable for whole-video recognition. In addressing the problem of facial recognition, the main contribution of this project is the design of a novel, trainable deep neural network framework that fuses spatial information and temporal information of video according to CNNs and LSTMs for pattern recognition. The experimental results on two public datasets, i.e., the RML and eNTERFACE05 databases, show that our framework outperforms previous state-of-the-art frameworks.INDEX TERMS Video-based facial expression recognition, CNNs, deep temporal-spatial features, optical flow, LSTM.

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The proposed method has 30 streams, i.e., 15 spatial streams and 15 temporal streams. Each spatial stream corresponds to each temporal stream. Therefore, this work correlates with the symmetry concept. It is a difficult task to classify video-based facial expression owing to the gap between the visual descriptors and the emotions. In order to bridge the gap, a new video descriptor for facial expression recognition is presented to aggregate spatial and temporal convolutional features across the entire extent of a video. The designed framework integrates a state-of-the-art 30 stream and has a trainable spatial–temporal feature aggregation layer. This framework is end-to-end trainable for video-based facial expression recognition. Thus, this framework can effectively avoid overfitting to the limited emotional video datasets, and the trainable strategy can learn to better represent an entire video. The different schemas for pooling spatial–temporal features are investigated, and the spatial and temporal streams are best aggregated by utilizing the proposed method. The extensive experiments on two public databases, BAUM-1s and eNTERFACE05, show that this framework has promising performance and outperforms the state-of-the-art strategies.

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