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SAFA: Sample-Adaptive Feature Augmentation for Long-Tailed Image Classification

Imbalanced datasets with long-tailed distribution widely exist in practice, posing great challenges for deep networks on how to handle the biased predictions between head (majority, frequent) classes and tail (minority, rare) classes. Feature space of tail classes learned by deep networks is usually under-represented, causing heterogeneous performance among different classes. Existing methods augment tail-class features to compensate tail classes on feature space, but these methods fail to generalize on test phase. To mitigate this problem, we propose a novel Sample-Adaptive Feature Augmentation (SAFA) to augment features for tail classes resulting in ameliorating the classifier performance. SAFA aims to extract diverse and transferable semantic directions from head classes, and adaptively translate tail-class features along extracted semantic directions for augmentation. SAFA leverages a recycling training scheme ensuring augmented features are sample-specific. Contrastive loss ensures the transferable semantic directions are class-irrelevant and mode seeking loss is adopted to produce diverse tail-class features and enlarge the feature space of tail classes. The proposed SAFA as a plug-in is convenient and versatile to be combined with different methods during training phase without additional computational burden at test time. By leveraging SAFA, we obtain outstanding results on CIFAR-LT-10, CIFAR-LT-100, Places-LT, ImageNet-LT, and iNaturalist2018.

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