  We present a novel unsupervised domain adaptation method for semantic
segmentation that generalizes a model trained with source images and
corresponding ground-truth labels to a target domain. A key to domain adaptive
semantic segmentation is to learn domain-invariant and discriminative features
without target ground-truth labels. To this end, we propose a bi-directional
pixel-prototype contrastive learning framework that minimizes intra-class
variations of features for the same object class, while maximizing inter-class
variations for different ones, regardless of domains. Specifically, our
framework aligns pixel-level features and a prototype of the same object class
in target and source images (i.e., positive pairs), respectively, sets them
apart for different classes (i.e., negative pairs), and performs the alignment
and separation processes toward the other direction with pixel-level features
in the source image and a prototype in the target image. The cross-domain
matching encourages domain-invariant feature representations, while the
bidirectional pixel-prototype correspondences aggregate features for the same
object class, providing discriminative features. To establish training pairs
for contrastive learning, we propose to generate dynamic pseudo labels of
target images using a non-parametric label transfer, that is, pixel-prototype
correspondences across different domains. We also present a calibration method
compensating class-wise domain biases of prototypes gradually during training.
