SGG-Nets: Generic Rotation-Invariant Plugin Networks for Point Cloud Analysis

Rotation invariance is a crucial requirement for the analysis of 3D point clouds. However, current methods often achieve rotation invariance by employing specific network designs. These networks, though perform well on rotation-aware tasks, is inferior in general tasks such as classification and segmentation. On the other hand, many powerful point processing networks, such as PointNet++, DGCNN, etc., have general point processing abilities, but do not own the property of rotation invariance. In this paper, we propose a standalone rotation-invariant convolution operator called SGGConv (Spherical Geometric Graph-based Convolution) and two ways integrating it with common point-based networks. The networks equipped with SGGConvs are called SGG-Nets which promote the rotation-invariance ability of regular point networks without modifying their network architectures much. Our contributions are three-fold. First, we propose a rotation-invariant feature descriptor, namely Spherical Geometry Descriptor (SGD), which captures point-pair features in a Local Spherical Coordinate System (LSCS). Second, we propose the SGGConv based on SGD and LSCS with an efficient Graph-based Spherical Feature Passing (GSFP) mechanism. Thirdly, we define two modules S-SGGConvMdl and M-SGGConvMdl, which are used to integrate SGGConv into baseline point nets. We test SGG-Nets, such as SGG-PointNet++, SGG-DGCNN, SGG-RIConv++, on representative point cloud datasets. These models, equipped with our SGGConvs, not only enhance the rotation-invariance of the baseline network but also improve its performance on point cloud analysis tasks such as classification and part segmentation, without incurring too much computational overhead.