  LiDAR point cloud analysis is a core task for 3D computer vision, especially
for autonomous driving. However, due to the severe sparsity and noise
interference in the single sweep LiDAR point cloud, the accurate semantic
segmentation is non-trivial to achieve. In this paper, we propose a novel
sparse LiDAR point cloud semantic segmentation framework assisted by learned
contextual shape priors. In practice, an initial semantic segmentation (SS) of
a single sweep point cloud can be achieved by any appealing network and then
flows into the semantic scene completion (SSC) module as the input. By merging
multiple frames in the LiDAR sequence as supervision, the optimized SSC module
has learned the contextual shape priors from sequential LiDAR data, completing
the sparse single sweep point cloud to the dense one. Thus, it inherently
improves SS optimization through fully end-to-end training. Besides, a
Point-Voxel Interaction (PVI) module is proposed to further enhance the
knowledge fusion between SS and SSC tasks, i.e., promoting the interaction of
incomplete local geometry of point cloud and complete voxel-wise global
structure. Furthermore, the auxiliary SSC and PVI modules can be discarded
during inference without extra burden for SS. Extensive experiments confirm
that our JS3C-Net achieves superior performance on both SemanticKITTI and
SemanticPOSS benchmarks, i.e., 4% and 3% improvement correspondingly.
