  LiDAR-based localization approach is a fundamental module for large-scale
navigation tasks, such as last-mile delivery and autonomous driving, and
localization robustness highly relies on viewpoints and 3D feature extraction.
Our previous work provides a viewpoint-invariant descriptor to deal with
viewpoint differences; however, the global descriptor suffers from a low
signal-noise ratio in unsupervised clustering, reducing the distinguishable
feature extraction ability. We develop SphereVLAD++, an attention-enhanced
viewpoint invariant place recognition method in this work. SphereVLAD++
projects the point cloud on the spherical perspective for each unique area and
captures the contextual connections between local features and their
dependencies with global 3D geometry distribution. In return, clustered
elements within the global descriptor are conditioned on local and global
geometries and support the original viewpoint-invariant property of SphereVLAD.
In the experiments, we evaluated the localization performance of SphereVLAD++
on both public KITTI360 datasets and self-generated datasets from the city of
Pittsburgh. The experiment results show that SphereVLAD++ outperforms all
relative state-of-the-art 3D place recognition methods under small or even
totally reversed viewpoint differences and shows 0.69% and 15.81% successful
retrieval rates with better than the second best. Low computation requirements
and high time efficiency also help its application for low-cost robots.
