PCLDet: Prototypical Contrastive Learning for Fine-Grained Object Detection in Remote Sensing Images

The capacity of satellites to supply high-resolution imaging has promoted the fine-grained object detection task in remote sensing images. However, this type of object detection is challenging due to low interclass feature differences in objects. To address this issue, we propose a prototypical contrastive learning-based detector (PCLDet) for fine-grained object detection in remote sensing images. The PCLDet first introduces the prototype to learn the fine-grained objects' features, and then adopts contrastive learning to compare the target and the learned features, thus improving the differentiability of the fine-grained object. Specifically, we first introduce the prototype, which represents the feature centers of each class, and then construct a prototype bank to store the feature prototypes of each class. Then, we introduce contrastive learning to extract the discriminative features by maximizing the interclass distance and minimizing the intraclass distance. Furthermore, we propose the ProtoCL loss as a part of the model optimization, which enables more representative prototypes to be learned. Finally, to address the long-tail problem in the remote sensing fine-grained object detection dataset, we propose a new proposal sampler, the class-balanced sampler (CBS) that can sample each class equally. Extensive experiments demonstrate that our method can achieve state-of-the-art performance on a commonly used aerial fine-grained object dataset (Fair1M) and aerial fine-grained ship dataset (OFSD) while maintaining high efficiency. The code will be available at https://github.com/G-Naughty/PCLDet.