Recall Loss for Imbalanced Image Classification and Semantic Segmentation
Authors: Junjiao Tian, Niluthpol Chowdhury Mithun, Zachary Seymour, Han-pang Chiu, Zsolt Kira
Keywords: Data Imbalance, Classification, Semantic Segmentation, Deep Learning
Abstract: Class imbalance is a fundamental problem in computer vision applications such as semantic segmentation and image classification. Specifically, uneven class distributions in a training dataset often result in unsatisfactory performance on under-represented classes. Many works have proposed to weigh the standard cross entropy loss function with pre-computed weights based on class statistics such as the number of samples and class margins. There are two major drawbacks to these methods: 1) constantly up-weighing minority classes can introduce excessive false positives especially in semantic segmentation; 2) many recent works discovered that pre-computed weights have adversarial effects on representation learning. In this regard, we propose a hard-class mining loss by reshaping the vanilla cross entropy loss such that it weights the loss for each class dynamically based on changing recall performance. We show mathematically that the novel recall loss changes gradually between the standard cross entropy loss and the well-known inverse frequency cross entropy loss and balances precision and accuracy. We first demonstrate that the proposed loss effectively balances precision and accuracy on semantic segmentation datasets, and leads to significant performance improvement over other state-of-the-art loss functions used in semantic segmentation, especially on shallow networks. On image classification, we design a simple two-head training strategy to show that the novel loss function improves representation learning on imbalanced datasets. We outperform the previously best performing method by $5.7\%$ on Place365-LT and by $1.1\%$ on iNaturalist.