Point cloud analysis without pose priors is very challenging in real applications, as the orientations of point clouds are often unknown. In this paper, we propose a brand new point-set learning framework PRIN, namely, Point-wise Rotation Invariant Network, focusing on rotation invariant feature extraction in point clouds analysis. We construct spherical signals by Density Aware Adaptive Sampling to deal with distorted point distributions in spherical space. Spherical Voxel Convolution and Point Re-sampling are proposed to extract rotation invariant features for each point. In addition, we extend PRIN to a sparse version called SPRIN, which directly operates on sparse point clouds. Both PRIN and SPRIN can be applied to tasks ranging from object classification, part segmentation, to 3D feature matching and label alignment. Results show that, on the dataset with randomly rotated point clouds, SPRIN demonstrates better performance than state-of-the-art methods without any data augmentation. We also provide thorough theoretical proof and analysis for point-wise rotation invariance achieved by our methods. Our code is available on https://github.com/qq456cvb/SPRIN.

本文提出了一个全新的点集学习框架PRIN，即点式旋转不变网络，通过密度感知自适应抽样构建球形信号来解决点云在球形空间上的扭曲分布，运用球形体素卷积和点重采样来提取每个点的旋转不变特征，同时将PRIN扩展到一种稀疏版本SPRIN，直接操作于稀疏点云。两种方法都适用于从对象分类、部分分割到三维特征匹配和标签对齐等任务，结果表明，对于随机旋转点云的数据集，SPRIN实现了比最先进的方法更好的性能，且无需使用任何数据增强。我们还为方法实现的点式旋转不变性提供了深入的理论证明和分析。

PRIN/SPRIN: 关于提取点旋转不变特征的研究