MLFA: Toward Realistic Test Time Adaptive Object Detection by Multi-Level Feature Alignment.

Object detection methods have achieved remarkable performances when the training and testing data satisfy the assumption of i.i.d. However, the training and testing data may be collected from different domains, and the gap between the domains can significantly degrade the detectors. Test Time Adaptive Object Detection (TTA-OD) is a novel online approach that aims to adapt detectors quickly and make predictions during the testing procedure. TTA-OD is more realistic than the existing unsupervised domain adaptation and source-free unsupervised domain adaptation approaches. For example, self-driving cars need to improve their perception of new environments in the TTA-OD paradigm during driving. To address this, we propose a multi-level feature alignment (MLFA) method for TTA-OD, which is able to adapt the model online based on the steaming target domain data. For a more straightforward adaptation, we select informative foreground and background features from image feature maps and capture their distributions using probabilistic models. Our approach includes: i) global-level feature alignment to align all informative feature distributions, thereby encouraging detectors to extract domain-invariant features, and ii) cluster-level feature alignment to match feature distributions for each category cluster across different domains. Through the multi-level alignment, we can prompt detectors to extract domain-invariant features, as well as align the category-specific components of image features from distinct domains. We conduct extensive experiments to verify the effectiveness of our proposed method. Our code is accessible at https://github.com/yaboliudotug/MLFA.