  Weakly-Supervised Temporal Action Localization (WSTAL) aims to localize
actions in untrimmed videos with only video-level labels. Currently, most
state-of-the-art WSTAL methods follow a Multi-Instance Learning (MIL) pipeline:
producing snippet-level predictions first and then aggregating to the
video-level prediction. However, we argue that existing methods have overlooked
two important drawbacks: 1) inadequate use of motion information and 2) the
incompatibility of prevailing cross-entropy training loss. In this paper, we
analyze that the motion cues behind the optical flow features are complementary
informative. Inspired by this, we propose to build a context-dependent motion
prior, termed as motionness. Specifically, a motion graph is introduced to
model motionness based on the local motion carrier (e.g., optical flow). In
addition, to highlight more informative video snippets, a motion-guided loss is
proposed to modulate the network training conditioned on motionness scores.
Extensive ablation studies confirm that motionness efficaciously models
action-of-interest, and the motion-guided loss leads to more accurate results.
Besides, our motion-guided loss is a plug-and-play loss function and is
applicable with existing WSTAL methods. Without loss of generality, based on
the standard MIL pipeline, our method achieves new state-of-the-art performance
on three challenging benchmarks, including THUMOS'14, ActivityNet v1.2 and
v1.3.
