I. Introduction
Point cloud registration is a transformation prediction problem to align two point clouds collected from the same object or scene. Point cloud registration has been widely used in many computer vision fields, such as 3D reconstruction, pose estimation, and self-localisation and mapping (SLAM) [1]. With the development of 3D sensors such as LiDAR and RGB-D cameras, point cloud registration has attracted more and more attention and many research works have been proposed in this field.