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Title: VideoEpitoma: Efficient Recognition of Long-range Actions

Paper content:
CNNs are widely successful in recognizing human actions in videos, albeit with a great cost of computation.
This cost is significantly higher in the case of long-range actions, where a video can span up to a few minutes, on average.
The goal of this paper is to reduce the computational cost of these CNNs, without sacrificing their performance.
We propose VideoEpitoma,  a neural network architecture comprising two modules: a timestamp selector and a video classifier.
Given a long-range video of thousands of timesteps, the selector learns to choose only a few but most representative timesteps for the video.
This selector resides on top of a lightweight CNN such as MobileNet and uses a novel gating module to take a binary decision: consider or discard a video timestep.
This decision is conditioned on both the timestep-level feature and the video-level consensus.
A heavyweight CNN model such as I3D takes the selected frames as input and performs video classification.
Using off-the-shelf video classifiers, VideoEpitoma reduces the computation by up to 50\% without compromising the accuracy.
In addition, we show that if trained end-to-end, the selector learns to make better choices to the benefit of the classifier, despite the selector and the classifier residing on two different CNNs.
Finally, we report state-of-the-art results on two datasets for long-range action recognition: Charades and Breakfast Actions, with much-reduced computation.
In particular, we match the accuracy of I3D by using less than half of the computation.
A human can skim through a minute-long video in just a few seconds, and still grasp its underlying story (Szelag et al., 2004) .
This extreme efficiency of the human visual and temporal information processing beggars belief.
The unmatched trade-off between efficiency and accuracy can be attributed to visual attention (Szelag et al., 2004 ) -one of the hallmarks of the human cognitive abilities.
This raises the question: can we build an efficient, yet effective, neural model to recognize minutes-long actions in videos?
A possible solution is building efficient neural networks, which have a demonstrated record of success in the efficient recognition of images (Howard et al., 2017) .
Such models have been successful for recognizing short-range actions in datasets such as HMDB (Kuehne et al., 2011) and UCF-101 (Soomro et al., 2012) , where analysis of only a few frames would suffice (Schindler & Van Gool, 2008) .
In contrast, a long-range action can take up to a few minutes to unfold (Hussein et al., 2019a) .
Current methods fully process the long-range action video to successfully recognize it.
Thus, for long-range actions, the major computational bottleneck is the sheer number of video frames to be processed.
Another potential solution is attention.
Not only it is biologically plausible, but also it is used in a wide spectrum of computer vision tasks, such as image classification , semantic segmentation (Oktay et al., 2018) , action recognition (Wang et al., 2018) and temporal localization (Nguyen et al., 2018) .
Attention has also been applied to language understanding (Lin et al., 2017) and graph modeling (Veličković et al., 2017) .
Most of these methods use soft-attention, where the insignificant visual signals are least attended to.
However, such signals are still fully processed by the neural network and hence no reduction on the computation cost is obtained.
Neural gating is a more conceivable choice to realize the efficiency, by completely dropping the insignificant visual signals.
Recently, there has been a notable success in making neural gating differentiable (Maddison et al., 2016) .
Neural gating is applied to conditional learning, and is used to gate network layers (Veit & Belongie, 2018) , convolutional channels (Bejnordi et al., 2019) , and more (Shetty et al., 2017) .
That begs the question: can neural gating help in reducing the computational cost of recognizing minutes-long actions?
That is to say, can we learn a gating mechanism to consider or discard video frames, conditioned on their video?
Motivated by the aforementioned questions, we propose VideoEpitoma, a two-stage neural network for efficient classification of long-range actions without compromising the performance.
The first stage is the timestep selector, in which, many timesteps of a long-range action are efficiently represented by lightweight CNN, such as MobileNet (Howard et al., 2017; Sandler et al., 2018; Howard et al., 2019) .
Then, a novel gating module learns to select only the most significant timesteps -practically achieving the epitoma (Latin for summary) of this video.
In the second stage, a heavyweight CNN, such as I3D (Carreira & Zisserman, 2017) , is used to effectively represent only the selected timesteps, followed by temporal modeling for the video-level recognition.
This paper contributes the followings: i.
VideoEpitoma, a neural network model for efficient recognition of long-range actions.
The proposed model uses a novel gating module for timestep selection, conditioned on both the input frame and its context.
ii.
Off the shelf, our timestamp selector benefits video classification models and yields signification reduction in computation costs.
We also show that if trained end-to-end, the timestep selector learns better gating mechanism to the benefit of the video classifier.
iii.
We present state-of-the-art results on two long-range action recognition benchmarks: Charades (Sigurdsson et al., 2016) and Breakfast Actions (Kuehne et al., 2014) with significant reductions in the computational costs.
In this paper, we proposed VideoEpitoma, a neural model for efficient recognition of long-range actions in videos.
We stated the fundamental differences between long-range actions and their shortrange counterparts (Hussein et al., 2019a; b) .
And we highlighted how these differences influenced our way of find a solution for an efficient recognition of such videos.
The outcome of this paper is VideoEpitoma, a neural model with the ability to retain the performance of off-the-shelf CNN classifiers at a fraction of the computational budget.
This paper concludes the following.
Rather than building an efficient CNN video classifier, we opted for an efficient selection of the most salient parts of the video, followed by an effective classification of only these salient parts.
For a successful selection, we proposed a novel gating module, able to select timesteps conditioned on their importance to their video.
We experimented how this selection benefits off-the-shelf CNN classifiers.
Futher more, we showed how VideoEpitoma, i.e. both the selector and the classifier, improves even further when trained end-to-end.
Finally, we experimented VideoEpitoma on two benchmarks for longrange actions.
We compared against realted methods to hightight the efficiency of videoEpitoma for saving the computation, and its effectiveness of recognizing the long-range actions.

assistant: text: Efficient video classification using frame-based conditional gating module for selecting most-dominant frames, followed by temporal modeling and classifier.