Mutual Learning of Joint and Separate Domain Alignments for Multi-Source Domain Adaptation

Multi-Source Domain Adaptation (MSDA) aims at transferring knowledge from multiple labeled source domains to benefit the task in an unlabeled target domain. The challenges of MSDA lie in mitigating domain gaps and combining information from diverse source domains. In most existing methods, the multiple source domains can be jointly or separately aligned to the target domain. In this work, we consider that these two types of methods, i.e. joint and separate domain alignments, are complementary and propose a mutual learning based alignment network (MLAN) to combine their advantages. Specifically, our proposed method is composed of three components, i.e. a joint alignment branch, a separate alignment branch, and a mutual learning objective between them. In the joint alignment branch, the samples from all source domains and the target domain are aligned together, with a single domain alignment goal, while in the separate alignment branch, each source domain is individually aligned to the target domain. Finally, by taking advantage of the complementarity of joint and separate domain alignment mechanisms, mutual learning is used to make the two branches learn collaboratively. Compared with other existing methods, our proposed MLAN integrates information of different domain alignment mechanisms and thus can mine rich knowledge from multiple domains for better performance. The experiments on DomainNet, Office-31, and Digits-five datasets demonstrate the effectiveness of our method.