PaCL: Part-level Contrastive Learning for Fine-grained Few-shot Image Classification

Recently, it is gaining increasingly attention to incorporate self-supervised technologies into few-shot learning. Previous methods have exclusively focused on image-level self-supervision, but they ignore that capturing subtle part features plays an important role in distinguishing fine-grained images. In this paper, we propose an approach named PaCL that embeds part-level contrastive learning into fine-grained few-shot image classification, strengthening the models' capability to extract discriminative features from indistinguishable images. PaCL treats parts as the inputs of contrastive learning, and it uses a transformation module to involve image-specific information into pre-defined meta parts, generating multiple features from each meta part depending on different images. To alleviate the impact of changes in views or occlusions, we propose to adopt part prototypes in contrastive learning. Part prototypes are generated by aggregating the features of each certain type of part, which are more reliable than directly using part features. A few-shot classifier is adopted to predict query images, which calculates the classification loss to optimize the transformation module and meta parts in conjunction with the loss calculated in contrastive learning. The optimization process will enforce the model to learn to extract discriminative and diverse features from different parts of the objects, even for the samples of unseen classes. Extensive studies show that our proposed method improves the performance of fine-grained few-shot image classification across several backbones, datasets, and tasks, achieving superior results compared with state-of-the-art methods.