FGRFlow: Learning Fine-Grained Rigidity Scene Flow from 4D Radar Point Cloud

Scene flow estimation using 4D millimeter-wave radar has emerged as a prominent research focus for 3D dynamic perception. However, compared to LiDAR point clouds, the drastic sparsity of radar point clouds poses challenges in enforcing local rigidity constraints, which are crucial for accurate 3D motion estimation. To address this issue, we propose a novel Gaussian-based pseudo-point generation method that fully leverages two distinct yet complementary data modalities, 3D coordinates and Doppler velocity, to support multi-body rigidity assumptions, effectively capturing fine-grained and structured motion patterns from highly sparse radar point clouds. Furthermore, a velocity calibration mechanism is designed to improve the reliability of fine-grained rigid motion velocity estimation. In addition, a progressive fusion strategy is introduced to systematically integrate fine-grained rigid motion priors at multiple levels, enhancing the robustness of matching costs and motion features while effectively compensating for coarse flows. Experimental results on real-world radar scans from the View-of-Delft (VoD) dataset demonstrate the promising performance of our FGRFlow compared to other leading 4D radar-based approaches, validating the advantages of our design choices.