  Recently unsupervised domain adaptation for the semantic segmentation task
has become more and more popular due to high-cost of pixel-level annotation on
real-world images. However, most domain adaptation methods are only restricted
to single-source-single-target pair, and can not be directly extended to
multiple target domains. In this work, we propose a collaborative learning
framework to achieve unsupervised multi-target domain adaptation. An
unsupervised domain adaptation expert model is first trained for each
source-target pair and is further encouraged to collaborate with each other
through a bridge built between different target domains. These expert models
are further improved by adding the regularization of making the consistent
pixel-wise prediction for each sample with the same structured context. To
obtain a single model that works across multiple target domains, we propose to
simultaneously learn a student model which is trained to not only imitate the
output of each expert on the corresponding target domain, but also to pull
different expert close to each other with regularization on their weights.
Extensive experiments demonstrate that the proposed method can effectively
exploit rich structured information contained in both labeled source domain and
multiple unlabeled target domains. Not only does it perform well across
multiple target domains but also performs favorably against state-of-the-art
unsupervised domain adaptation methods specially trained on a single
source-target pair
