  In the pursuit of transferring a source model to a target domain without
access to the source training data, Source-Free Domain Adaptation (SFDA) has
been extensively explored across various scenarios, including closed-set,
open-set, partial-set, and generalized settings. Existing methods, focusing on
specific scenarios, not only address only a subset of challenges but also
necessitate prior knowledge of the target domain, significantly limiting their
practical utility and deployability. In light of these considerations, we
introduce a more practical yet challenging problem, termed unified SFDA, which
comprehensively incorporates all specific scenarios in a unified manner. To
tackle this unified SFDA problem, we propose a novel approach called Latent
Causal Factors Discovery (LCFD). In contrast to previous alternatives that
emphasize learning the statistical description of reality, we formulate LCFD
from a causality perspective. The objective is to uncover the causal
relationships between latent variables and model decisions, enhancing the
reliability and robustness of the learned model against domain shifts. To
integrate extensive world knowledge, we leverage a pre-trained vision-language
model such as CLIP. This aids in the formation and discovery of latent causal
factors in the absence of supervision in the variation of distribution and
semantics, coupled with a newly designed information bottleneck with
theoretical guarantees. Extensive experiments demonstrate that LCFD can achieve
new state-of-the-art results in distinct SFDA settings, as well as source-free
out-of-distribution generalization.Our code and data are available at
https://github.com/tntek/source-free-domain-adaptation.
