Weakly-supervised temporal action localization aims to detect intervals of action instances with only video-level action labels for training. A crucial challenge is to separate frames of action classes from remaining, denoted as background frames (i.e., frames not belonging to any action class). Previous methods attempt background modeling by either synthesizing pseudo background videos with static frames or introducing an auxiliary class for background. However, they overlook an essential fact that background frames could be dynamic and inconsistent. Accordingly, we cast the problem of identifying background frames as out-of-distribution detection and isolate it from conventional action classification. Beyond our base action localization network, we propose a module to estimate the probability of being background (i.e., uncertainty [20]), which allows us to learn uncertainty given only video-level labels via multiple instance learning. A background entropy loss is further designed to reject background frames by forcing them to have uniform probability distribution for action classes. Extensive experiments verify the effectiveness of our background modeling and show that our method significantly outperforms state-of-the-art methods on the standard benchmarks - THUMOS'14 and ActivityNet (1.2 and 1.3). Our code and the trained model are available at https://github.com/Pilhyeon/Background-Modeling-via-Uncertainty-Estimation.

该论文提出了一种新的关于背景帧的观点，在这个观点中，背景帧是关于其不一致性的离群样本，通过学习不确定性，背景帧可以被检测出来，而这种不确定性的学习是通过多实例学习实现的，进一步引入了一个背景熵损失以更好地鉴别背景帧。研究表明，我们的不确定性建模可以有效地减轻背景帧的干扰，并且在THUMOS'14和ActivityNet（1.2和1.3）基准测试上取得了明显优于最先进方法的性能表现。

基于不确定性建模进行弱监督时序动作定位