Abstract: We consider the task of identifying human actions visible in online videos. We focus on the widely spread genre of lifestyle vlogs, which consist of videos of people performing actions while verbally describing them. Our goal is to identify if actions mentioned in the speech description of a video are visually present. We construct a dataset with crowdsourced manual annotations of visible actions, and introduce a multimodal algorithm that leverages information derived from visual and linguistic clues to automatically infer which actions are visible in a video. We demonstrate that our multimodal algorithm outperforms algorithms based only on one modality at a time.

Introduction: There has been a surge of recent interest in detecting human actions in videos. Work in this space has mainly focused on learning actions from articulated human pose BIBREF7 , BIBREF8 , BIBREF9 or mining spatial and temporal information from videos BIBREF10 , BIBREF11 . A number of resources have been produced, including Action Bank BIBREF12 , NTU RGB+D BIBREF13 , SBU Kinect Interaction BIBREF14 , and PKU-MMD BIBREF15 . Most research on video action detection has gathered video information for a set of pre-defined actions BIBREF2 , BIBREF16 , BIBREF1 , an approach known as explicit data gathering BIBREF0 . For instance, given an action such as “open door,” a system would identify videos that include a visual depiction of this action. While this approach is able to detect a specific set of actions, whose choice may be guided by downstream applications, it achieves high precision at the cost of low recall. In many cases, the set of predefined actions is small (e.g., 203 activity classes in BIBREF2 ), and for some actions, the number of visual depictions is very small. An alternative approach is to start with a set of videos, and identify all the actions present in these videos BIBREF17 , BIBREF18 . This approach has been referred to as implicit data gathering, and it typically leads to the identification of a larger number of actions, possibly with a small number of examples per action. In this paper, we use an implicit data gathering approach to label human activities in videos. To the best of our knowledge, we are the first to explore video action recognition using both transcribed audio and video information. We focus on the popular genre of lifestyle vlogs, which consist of videos of people demonstrating routine actions while verbally describing them. We use these videos to develop methods to identify if actions are visually present. The paper makes three main contributions. First, we introduce a novel dataset consisting of 1,268 short video clips paired with sets of actions mentioned in the video transcripts, as well as manual annotations of whether the actions are visible or not. The dataset includes a total of 14,769 actions, 4,340 of which are visible. Second, we propose a set of strong baselines to determine whether an action is visible or not. Third, we introduce a multimodal neural architecture that combines information drawn from visual and linguistic clues, and show that it improves over models that rely on one modality at a time. By making progress towards automatic action recognition, in addition to contributing to video understanding, this work has a number of important and exciting applications, including sports analytics BIBREF19 , human-computer interaction BIBREF20 , and automatic analysis of surveillance video footage BIBREF21 . The paper is organized as follows. We begin by discussing related work, then describe our data collection and annotation process. We next overview our experimental set-up and introduce a multimodal method for identifying visible actions in videos. Finally, we discuss our results and conclude with general directions for future work.

Question: How many videos did they use?
Answer: 177