  Point-level supervised temporal action localization (PTAL) aims at
recognizing and localizing actions in untrimmed videos where only a single
point (frame) within every action instance is annotated in training data.
Without temporal annotations, most previous works adopt the multiple instance
learning (MIL) framework, where the input video is segmented into
non-overlapped short snippets, and action classification is performed
independently on every short snippet. We argue that the MIL framework is
suboptimal for PTAL because it operates on separated short snippets that
contain limited temporal information. Therefore, the classifier only focuses on
several easy-to-distinguish snippets instead of discovering the whole action
instance without missing any relevant snippets. To alleviate this problem, we
propose a novel method that localizes actions by generating and evaluating
action proposals of flexible duration that involve more comprehensive temporal
information. Moreover, we introduce an efficient clustering algorithm to
efficiently generate dense pseudo labels that provide stronger supervision, and
a fine-grained contrastive loss to further refine the quality of pseudo labels.
Experiments show that our proposed method achieves competitive or superior
performance to the state-of-the-art methods and some fully-supervised methods
on four benchmarks: ActivityNet 1.3, THUMOS 14, GTEA, and BEOID datasets.
