  Recent progresses in domain adaptive semantic segmentation demonstrate the
effectiveness of adversarial learning (AL) in unsupervised domain adaptation.
However, most adversarial learning based methods align source and target
distributions at a global image level but neglect the inconsistency around
local image regions. This paper presents a novel multi-level adversarial
network (MLAN) that aims to address inter-domain inconsistency at both global
image level and local region level optimally. MLAN has two novel designs,
namely, region-level adversarial learning (RL-AL) and co-regularized
adversarial learning (CR-AL). Specifically, RL-AL models prototypical regional
context-relations explicitly in the feature space of a labelled source domain
and transfers them to an unlabelled target domain via adversarial learning.
CR-AL fuses region-level AL and image-level AL optimally via mutual
regularization. In addition, we design a multi-level consistency map that can
guide domain adaptation in both input space ($i.e.$, image-to-image
translation) and output space ($i.e.$, self-training) effectively. Extensive
experiments show that MLAN outperforms the state-of-the-art with a large margin
consistently across multiple datasets.
