  Incorporating multiple camera views for detection alleviates the impact of
occlusions in crowded scenes. In a multiview system, we need to answer two
important questions when dealing with ambiguities that arise from occlusions.
First, how should we aggregate cues from the multiple views? Second, how should
we aggregate unreliable 2D and 3D spatial information that has been tainted by
occlusions? To address these questions, we propose a novel multiview detection
system, MVDet. For multiview aggregation, existing methods combine anchor box
features from the image plane, which potentially limits performance due to
inaccurate anchor box shapes and sizes. In contrast, we take an anchor-free
approach to aggregate multiview information by projecting feature maps onto the
ground plane (bird's eye view). To resolve any remaining spatial ambiguity, we
apply large kernel convolutions on the ground plane feature map and infer
locations from detection peaks. Our entire model is end-to-end learnable and
achieves 88.2% MODA on the standard Wildtrack dataset, outperforming the
state-of-the-art by 14.1%. We also provide detailed analysis of MVDet on a
newly introduced synthetic dataset, MultiviewX, which allows us to control the
level of occlusion. Code and MultiviewX dataset are available at
https://github.com/hou-yz/MVDet.
