Mask Focal Loss: A unifying framework for dense crowd counting with canonical object detection networks

As a fundamental computer vision task, crowd counting predicts the number of pedestrians in a scene, which plays an important role in risk perception and early warning, traffic control and scene statistical analysis. Currently, deep learning based head detection is a promising method for crowd counting. However, the highly concerned object detection networks cannot be well applied to this field for three reasons: (1) The sample imbalance has not been overcome yet in highly dense and complex scenes because the existing loss functions calculate the positive loss at a single key point or in the entire target area with the same weight for all pixels; (2) The canonical object detectors' loss calculation is a hard assignment without taking into account the space coherence from the object location to the background region; and (3) Most of the existing head detection datasets are only annotated with the center points instead of bounding boxes which is mandatory for the canonical detectors. To address these problems, we propose a novel loss function, called Mask Focal Loss (MFL), to redefine the loss contributions according to the situ value of the heatmap with a Gaussian kernel. MFL provides a unifying framework for the loss functions based on both heatmap and binary feature map ground truths. Meanwhile, for better evaluation and comparison, a new synthetic dataset GTA\_Head is built, including 35 sequences, 5096 images and 1732043 head labels with bounding boxes. Experimental results show the overwhelming performance and demonstrate that our proposed MFL framework is applicable to all of the canonical detectors and to various datasets with different annotation patterns. This work provides a strong baseline for surpassing the crowd counting methods based on density estimation.