Reconciling Object-Level and Global-Level Objectives for Long-Tail Detection

Large vocabulary object detectors are often faced with the long-tailed label distributions, seriously degrading their ability to detect rarely seen categories. On one hand, the rare objects are prone to be misclassified as frequent categories. On the other hand, due to the limitation on the total number of detections per image, detectors usually rank all the confidence scores globally and filter out the lower-ranking ones. This may result in missed detection during inference, especially for the rare categories that naturally come with lower scores. Existing methods mainly focus on the former problem and design various classification loss to enhance the object-level classification accuracy, but largely overlook the global-level ranking task. In this paper, we propose a novel framework that Reconciles Object-level and Global-level (ROG) objectives to address both problems. As a multi-task learning framework, ROG simultaneously trains the model with two tasks: classifying each object proposal individually and ranking all the confidence scores globally. Specifically, complementary to the object-level classification loss for model discrimination, we design a generalized average precision (GAP) loss to explicitly optimize the global-level score ranking across different objects. For each category, GAP loss generates balanced gradients to rectify the ranking errors. In experiments, we show that GAP loss is highly versatile to be plugged into various advanced methods and brings considerable benefits.