Effective spatiotemporal feature representation is crucial to the video-based action recognition task. Focusing on discriminate spatiotemporal feature learning, we propose Information Fused Temporal Transformation Network (IF-TTN) for action recognition on top of popular Temporal Segment Network (TSN) framework. In the network, Information Fusion Module (IFM) is designed to fuse the appearance and motion features at multiple ConvNet levels for each video snippet, forming a short-term video descriptor. With fused features as inputs, Temporal Transformation Networks (TTN) are employed to model middle-term temporal transformation between the neighboring snippets following a sequential order. As TSN itself depicts long-term temporal structure by segmental consensus, the proposed network comprehensively considers multiple granularity temporal features. Our IF-TTN achieves the state-of-the-art results on two most popular action recognition datasets: UCF101 and HMDB51. Empirical investigation reveals that our architecture is robust to the input motion map quality. Replacing optical flow with the motion vectors from compressed video stream, the performance is still comparable to the flow-based methods while the testing speed is 10x faster.
State-of-the-art video understanding methods relied heavily on optical flow. The heavy computation cost of optical flow prevented these methods from real-time implementation. There were a few works dealt with real-time video understanding by replacing the costly optical flow with low-cost motion representations. Bilen proposed dynamic image (DI) @cite_31 to simulate the motion information, and Sun proposed Optical Flow guided Feature (OFF) @cite_22 to model short-term temporal variation (e.g. at a temporal length of about 7 frames 7 frames are calculated from the training strategy of OFF. TSN Two-Stream CNN used 5 stacked optical flow frames to model short-term motions. Thus 7 frames belong to short-term motions. ). Motion Vector (MV) was a coarse representation of motion, but it can be obtained directly from compressed video streams without extra calculation. Therefore, Enhanced Motion Vectors CNN (EMV-CNN) @cite_18 used motion vector as the input of temporal CNN to improve inference speed and CoViAR @cite_1 adopted an accumulated motion vector for real-time action recognition. Suffered from the lack of fine detailed motion information in MV, recognition performance was degraded dramatically. The performance of both EMV-CNN and CoViAR was far behind Two-Stream CNN with optical flow.
@cite_1 @cite_31 @cite_18 @cite_22
2772539318 2462996230 2774324727
Training robust deep video representations has proven to be much more challenging than learning deep image representations and consequently hampered tasks like video action recognition. This is in part due to the enormous size of raw video streams, the associated amount of computation required, and the high temporal redundancy. The 'true' and interesting signal is often drowned in too much irrelevant data. Motivated by the fact that the superfluous information can be reduced by up to two orders of magnitude with video compression techniques (like H.264, HEVC, etc.), in this work, we propose to train a deep network directly on the compressed video, devoid of redundancy, rather than the traditional highly redundant RGB stream. This representation has a higher information density and we found the training to be easier. In addition, the signals in a compressed video provide free, albeit noisy, motion information. We propose novel techniques to use them effectively. Our approach is about 4.6 times faster than a state-of-the-art 3D-CNN model, 2.7 times faster than a ResNet-152, and very easy to implement. On the task of action recognition, our approach outperforms all the other methods on the UCF-101, HMDB-51, and Charades dataset. We introduce the concept of dynamic image, a novel compact representation of videos useful for video analysis especially when convolutional neural networks (CNNs) are used. The dynamic image is based on the rank pooling concept and is obtained through the parameters of a ranking machine that encodes the temporal evolution of the frames of the video. Dynamic images are obtained by directly applying rank pooling on the raw image pixels of a video producing a single RGB image per video. This idea is simple but powerful as it enables the use of existing CNN models directly on video data with fine-tuning. We present an efficient and effective approximate rank pooling operator, speeding it up orders of magnitude compared to rank pooling. Our new approximate rank pooling CNN layer allows us to generalize dynamic images to dynamic feature maps and we demonstrate the power of our new representations on standard benchmarks in action recognition achieving state-of-the-art performance. Motion representation plays a vital role in human action recognition in videos. In this study, we introduce a novel compact motion representation for video action recognition, named Optical Flow guided Feature (OFF), which enables the network to distill temporal information through a fast and robust approach. The OFF is derived from the definition of optical flow and is orthogonal to the optical flow. The derivation also provides theoretical support for using the difference between two frames. By directly calculating pixel-wise spatiotemporal gradients of the deep feature maps, the OFF could be embedded in any existing CNN based video action recognition framework with only a slight additional cost. It enables the CNN to extract spatiotemporal information, especially the temporal information between frames simultaneously. This simple but powerful idea is validated by experimental results. The network with OFF fed only by RGB inputs achieves a competitive accuracy of 93.3 on UCF-101, which is comparable with the result obtained by two streams (RGB and optical flow), but is 15 times faster in speed. Experimental results also show that OFF is complementary to other motion modalities such as optical flow. When the proposed method is plugged into the state-of-the-art video action recognition framework, it has 96:0 and 74:2 accuracy on UCF-101 and HMDB-51 respectively. The code for this project is available at this https URL.