  Existing temporal action detection (TAD) methods rely on a large number of
training data with segment-level annotations. Collecting and annotating such a
training set is thus highly expensive and unscalable. Semi-supervised TAD
(SS-TAD) alleviates this problem by leveraging unlabeled videos freely
available at scale. However, SS-TAD is also a much more challenging problem
than supervised TAD, and consequently much under-studied. Prior SS-TAD methods
directly combine an existing proposal-based TAD method and a SSL method. Due to
their sequential localization (e.g, proposal generation) and classification
design, they are prone to proposal error propagation. To overcome this
limitation, in this work we propose a novel Semi-supervised Temporal action
detection model based on PropOsal-free Temporal mask (SPOT) with a parallel
localization (mask generation) and classification architecture. Such a novel
design effectively eliminates the dependence between localization and
classification by cutting off the route for error propagation in-between. We
further introduce an interaction mechanism between classification and
localization for prediction refinement, and a new pretext task for
self-supervised model pre-training. Extensive experiments on two standard
benchmarks show that our SPOT outperforms state-of-the-art alternatives, often
by a large margin. The PyTorch implementation of SPOT is available at
https://github.com/sauradip/SPOT
