Active Learning for Open-Set Annotation Using Contrastive Query Strategy

Active learning has achieved remarkable success in minimizing labeling costs for classification tasks with all data samples drawn from known classes. However, in real scenarios, most active learning methods fail when encountering open-set annotation (OSA) problem, i.e., numerous samples from unknown classes. The main reason for such failure comes from existing query strategies that are unavoidable to select unknown class samples. To tackle such problem and select the most informative samples, we propose a novel active learning framework named OSA-CQ, which simplifies the detection work of samples from known classes and enhances the classification performance with an effective contrastive query strategy. Specifically, OSA-CQ firstly adopts an auxiliary network to distinguish samples using confidence scores, which can dynamically select samples with the highest probability from known classes in the unlabeled set. Secondly, by comparing the predictions between auxiliary network, classification, and feature similarity, OSA-CQ designs a contrastive query strategy to select these most informative samples from unlabeled and known classes set. Experimental results on CIFAR10, CIFAR100 and Tiny-ImageNet show the proposed OSA-CQ can select samples from known classes with high information, and achieve higher classification performance with lower annotation cost than state-of-the-art active learning algorithms.