SCMix: Stochastic Compound Mixing for Open Compound Domain Adaptation in Semantic Segmentation

Open compound domain adaptation (OCDA) aims to transfer knowledge from a labeled source domain to a mix of unlabeled homogeneous compound target domains while generalizing to open unseen domains. Existing OCDA methods solve the intradomain gaps by a divide-and-conquer strategy, which decomposes the problem into several individual and parallel domain adaptation (DA) tasks. In this work, starting from the general DA theory, we establish a novel generalization bound for the setting of OCDA. Built upon this, we argue that conventional OCDA approaches may substantially underestimate the inherent variance inside the compound target domains for model generalization, constraining the model's performance. We subsequently present stochastic compound mixing (SCMix), an augmentation strategy with the primary objective of mitigating the divergence between the source and mixed target distributions. Theoretical analyses are conducted to substantiate the superiority of SCMix, proving that single-target mixing is a subgroup of our method. Extensive experiments show that our method attains a lower empirical risk on OCDA semantic segmentation tasks, thus supporting our theories. In particular, combining the transformer architecture, SCMix achieves a notable performance boost compared to SoTA results.