Affinity3D: Propagating Instance-Level Semantic Affinity for Zero-Shot Point Cloud Semantic Segmentation

Zero-shot point cloud semantic segmentation aims to recognize novel classes at the point level. Previous methods mainly transfer excellent zero-shot generalization capabilities from images to point clouds. However, directly transferring knowledge from images to point clouds faces two ambiguous problems. On the one hand, 2D models will generate wrong predictions when the image changes. On the other hand, directly mapping 3D points to 2D pixels by perspective projection fails to consider the visibility of 3D points in camera view. The wrong geometric alignment of 3D points and 2D pixels causes semantic ambiguity. To tackle these two problems, we propose a framework named Affinity3D that intends to empower 3D semantic segmentation models to perceive novel samples. Our framework aggregates instances in 3D and recognizes them in 2D, leveraging the excellent geometric separation in 3D and the zero-shot capabilities of 2D models. Affinity3D involves an affinity module that rectifies the wrong predictions by comparing them with similar instances and a visibility module preventing knowledge transfer from visible 2D pixels to invisible 3D points. Extensive experiments have been conducted on the SemanticKITTI and nuScenes datasets. Our framework achieves state-of-the-art performance on both two datasets. Code is available at https://github.com/opjang5/Affinity3D.