  We address the problem of language-based temporal localization in untrimmed
videos. Compared to temporal localization with fixed categories, this problem
is more challenging as the language-based queries not only have no pre-defined
activity list but also may contain complex descriptions. Previous methods
address the problem by considering features from video sliding windows and
language queries and learning a subspace to encode their correlation, which
ignore rich semantic cues about activities in videos and queries. We propose to
mine activity concepts from both video and language modalities by applying the
actionness score enhanced Activity Concepts based Localizer (ACL).
Specifically, the novel ACL encodes the semantic concepts from verb-obj pairs
in language queries and leverages activity classifiers' prediction scores to
encode visual concepts. Besides, ACL also has the capability to regress sliding
windows as localization results. Experiments show that ACL significantly
outperforms state-of-the-arts under the widely used metric, with more than 5%
increase on both Charades-STA and TACoS datasets.
