  Automatic Crowd behavior analysis can be applied to effectively help the
daily transportation statistics and planning, which helps the smart city
construction. As one of the most important keys, crowd counting has drawn
increasing attention. Recent works achieved promising performance but relied on
the supervised paradigm with expensive crowd annotations. To alleviate the
annotation cost in real-world transportation scenarios, in this work we
proposed a semi-supervised learning framework $S^{4}\textit{Crowd}$, which can
leverage both unlabeled/labeled data for robust crowd counting. In the
unsupervised pathway, two \textit{self-supervised losses} were proposed to
simulate the crowd variations such as scale, illumination, based on which
supervised information pseudo labels were generated and gradually refined. We
also proposed a crowd-driven recurrent unit \textit{Gated-Crowd-Recurrent-Unit
(GCRU)}, which can preserve discriminant crowd information by extracting
second-order statistics, yielding pseudo labels with improved quality. A joint
loss including both unsupervised/supervised information was proposed, and a
dynamic weighting strategy was employed to balance the importance of the
unsupervised loss and supervised loss at different training stages. We
conducted extensive experiments on four popular crowd counting datasets in
semi-supervised settings. Experimental results supported the effectiveness of
each proposed component in our $S^{4}$Crowd framework. Our method achieved
competitive performance in semi-supervised learning approaches on these crowd
counting datasets.
