Multi-Level Bottom-Top and Top-Bottom Feature Fusion for Crowd Counting

Crowd counting presents enormous challenges in the form of large variation in scales within images and across the dataset. These issues are further exacerbated in highly congested scenes. Approaches based on straightforward fusion of multi-scale features from a deep network seem to be obvious solutions to this problem. However, these fusion approaches do not yield significant improvements in the case of crowd counting in congested scenes. This is usually due to their limited abilities in effectively combining the multi-scale features for problems like crowd counting. To overcome this, we focus on how to efficiently leverage information present in different layers of the network. Specifically, we present a network that involves: (i) a multi-level bottom-top and top-bottom fusion (MBTTBF) method to combine information from shallower to deeper layers and vice versa at multiple levels, (ii) scale complementary feature extraction blocks (SCFB) involving cross-scale residual functions to explicitly enable flow of complementary features from adjacent conv layers along the fusion paths. Furthermore, in order to increase the effectiveness of the multi-scale fusion, we employ a principled way of generating scale-aware ground-truth density maps for training. Experiments conducted on three datasets that contain highly congested scenes (ShanghaiTech, UCF_CC_50, and UCF-QNRF) demonstrate that the proposed method is able to outperform several recent methods in all the datasets.

1. Introduction

Computer vision-based crowd counting [8, 17, 26, 27, 36, 44, 48, 56, 68, 69, 74, 77] has witnessed tremendous progress in the recent years. Algorithms developed for crowd counting have found a variety of applications such as video and traffic surveillance [15, 21, 38, 59, 64, 71, 72], agriculture monitoring (plant counting) [35], cell counting [22], scene understanding, urban planning and environmental survey [11, 68].