Unsupervised domain adaptation aims to align a labeled source domain and an unlabeled target domain, but it requires to access the source data which often raises concerns in data privacy, data portability and data transmission efficiency. We study unsupervised model adaptation (UMA), or called Unsupervised Domain Adaptation without Source Data, an alternative setting that aims to adapt source-trained models towards target distributions without accessing source data. To this end, we design an innovative historical contrastive learning (HCL) technique that exploits historical source hypothesis to make up for the absence of source data in UMA. HCL addresses the UMA challenge from two perspectives. First, it introduces historical contrastive instance discrimination (HCID) that learns from target samples by contrasting their embeddings which are generated by the currently adapted model and the historical models. With the source-trained and earlier-epoch models as the historical models, HCID encourages UMA to learn instance-discriminative target representations while preserving the source hypothesis. Second, it introduces historical contrastive category discrimination (HCCD) that pseudo-labels target samples to learn category-discriminative target representations. Instead of globally thresholding pseudo labels, HCCD re-weights pseudo labels according to their prediction consistency across the current and historical models. Extensive experiments show that HCL outperforms and complements state-of-the-art methods consistently across a variety of visual tasks (e.g., segmentation, classification and detection) and setups (e.g., close-set, open-set and partial adaptation).

本研究提出了一种无监督模型自适应的历史对比学习技术，通过历史模型的假设来弥补源数据中的缺失，从而实现将源训练模型适应到目标分布的无监督域适应。实验结果表明，该方法在各种视觉任务和设置下一致优于现有技术。

模型适应：无源数据对比学习的无监督域自适应