The highest accuracy object detectors to date are based on a two-stage approach popularized by R-CNN, where a classifier is applied to a sparse set of candidate object locations. In contrast, one-stage detectors that are applied over a regular, dense sampling of possible object locations have the potential to be faster and simpler, but have trailed the accuracy of two-stage detectors thus far. In this paper, we investigate why this is the case. We discover that the extreme foreground-background class imbalance encountered during training of dense detectors is the central cause. We propose to address this class imbalance by reshaping the standard cross entropy loss such that it down-weights the loss assigned to well-classified examples. Our novel Focal Loss focuses training on a sparse set of hard examples and prevents the vast number of easy negatives from overwhelming the detector during training. To evaluate the effectiveness of our loss, we design and train a simple dense detector we call RetinaNet. Our results show that when trained with the focal loss, RetinaNet is able to match the speed of previous one-stage detectors while surpassing the accuracy of all existing state-of-the-art two-stage detectors.

本文研究了单阶段物体检测器比两阶段物体检测器速度更快，但精度更低的原因，并提出了一种名为Focal Loss的做法，以解决稠密检测器训练中遇到的前景背景类别不平衡问题，该方法能够提高准确性。在该方法的基础上设计了一种名为RetinaNet的密集型检测器，取得了超过现有首屈一指的两阶段检测器的精度。

密集目标检测的Focal Loss