We discuss the robustness and generalization ability in the realm of action recognition, showing that the mainstream neural networks are not robust to disordered frames and diverse video environments. There are two possible reasons: First, existing models lack an appropriate method to overcome the inevitable decision discrepancy between multiple streams with different input modalities. Second, by doing cross-dataset experiments, we find that the optical flow features are hard to be transferred, which affects the generalization ability of the two-stream neural networks. For robust action recognition, we present the Reversed Two-Stream Networks (Rev2Net) which has three properties: (1) It could learn more transferable, robust video features by reversing the multi-modality inputs as training supervisions. It outperforms all other compared models in challenging frames shuffle experiments and cross-dataset experiments. (2) It is highlighted by an adaptive, collaborative multi-task learning approach that is applied between decoders to penalize their disagreement in the deep feature space. We name it the decoding discrepancy penalty (DDP). (3) As the decoder streams will be removed at test time, Rev2Net makes recognition decisions purely based on raw video frames. Rev2Net achieves the best results in the cross-dataset settings and competitive results on classic action recognition tasks: 94.6 for UCF-101, 71.1 for HMDB-51 and 73.3 for Kinetics. It performs even better than most methods who take extra inputs beyond raw RGB frames.
The two-stream networks were first introduced to the deep video classification models by Simonyan @cite_24 which model short temporal snapshots of videos by averaging the predictions from a single RGB frame and a stack of computed optical flow frames. Noticing that RGB images could not fully exploit temporal cues, they extracted the optical flow from consecutive video frames and took them as the network inputs. The optical flow stream has brought in a significant performance gain. Since then, two-stream networks have been widely employed by many video action recognition models @cite_11 @cite_0 @cite_18 @cite_23 @cite_25 , including some 3D CNN models @cite_31 @cite_4 @cite_6 .
@cite_18 @cite_4 @cite_6 @cite_24 @cite_0 @cite_23 @cite_31 @cite_25 @cite_11
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The video and action classification have extremely evolved by deep neural networks specially with two stream CNN using RGB and optical flow as inputs and they present outstanding performance in terms of video analysis. One of the shortcoming of these methods is handling motion information extraction which is done out side of the CNNs and relatively time consuming also on GPUs. So proposing end-to-end methods which are exploring to learn motion representation, like 3D-CNN can achieve faster and accurate performance. We present some novel deep CNNs using 3D architecture to model actions and motion representation in an efficient way to be accurate and also as fast as real-time. Our new networks learn distinctive models to combine deep motion features into appearance model via learning optical flow features inside the network. Recognizing actions in videos is a challenging task as video is an information-intensive media with complex variations. Most existing methods have treated video as a flat data sequence while ignoring the intrinsic hierarchical structure of the video content. In particular, an action may span different granularities in this hierarchy including, from small to large, a single frame, consecutive frames (motion), a short clip, and the entire video. In this paper, we present a novel framework to boost action recognition by learning a deep spatio-temporal video representation at hierarchical multi-granularity. Specifically, we model each granularity as a single stream by 2D (for frame and motion streams) or 3D (for clip and video streams) convolutional neural networks (CNNs). The framework therefore consists of multi-stream 2D or 3D CNNs to learn both the spatial and temporal representations. Furthermore, we employ the Long Short-Term Memory (LSTM) networks on the frame, motion, and clip streams to exploit long-term temporal dynamics. With a softmax layer on the top of each stream, the classification scores can be predicted from all the streams, followed by a novel fusion scheme based on the multi-granular score distribution. Our networks are learned in an end-to-end fashion. On two video action benchmarks of UCF101 and HMDB51, our framework achieves promising performance compared with the state-of-the-art. The deep two-stream architecture [23] exhibited excellent performance on video based action recognition. The most computationally expensive step in this approach comes from the calculation of optical flow which prevents it to be real-time. This paper accelerates this architecture by replacing optical flow with motion vector which can be obtained directly from compressed videos without extra calculation. However, motion vector lacks fine structures, and contains noisy and inaccurate motion patterns, leading to the evident degradation of recognition performance. Our key insight for relieving this problem is that optical flow and motion vector are inherent correlated. Transferring the knowledge learned with optical flow CNN to motion vector CNN can significantly boost the performance of the latter. Specifically, we introduce three strategies for this, initialization transfer, supervision transfer and their combination. Experimental results show that our method achieves comparable recognition performance to the state-of-the-art, while our method can process 390.7 frames per second, which is 27 times faster than the original two-stream method. We investigate architectures of discriminatively trained deep Convolutional Networks (ConvNets) for action recognition in video. The challenge is to capture the complementary information on appearance from still frames and motion between frames. We also aim to generalise the best performing hand-crafted features within a data-driven learning framework. Our contribution is three-fold. First, we propose a two-stream ConvNet architecture which incorporates spatial and temporal networks. Second, we demonstrate that a ConvNet trained on multi-frame dense optical flow is able to achieve very good performance in spite of limited training data. Finally, we show that multitask learning, applied to two different action classification datasets, can be used to increase the amount of training data and improve the performance on both. Our architecture is trained and evaluated on the standard video actions benchmarks of UCF-101 and HMDB-51, where it is competitive with the state of the art. It also exceeds by a large margin previous attempts to use deep nets for video classification.