Drones as Annotators: Amodal 3D Auto-Labeling for Ground LiDAR with Aerial Priors
Scaling perception data in autonomous driving is hindered by manual 3D bounding box annotation, a costly and labor intensive process requiring substantial domain expertise. Existing auto-labeling methods reduce this burden, but most of them rely on onboard sensors, where a single ground-level viewpoint yields occluded and sparse observations and inaccurate object geometry. We introduce DAA (Drones as Annotators), a drone-assisted training-free framework for amodal 3D auto-labeling. DAA augments ground LiDAR with an aerial agent that provides less occluded vehicle detections with continuous tracks. Ground LiDAR, in turn, provides precise metric 3D geometry unavailable from aerial imagery alone. DAA exploits the two complementary yet heterogeneous modalities through a two-stage framework: (i) air-ground coordinate alignment unifies aerial detections and ground LiDAR scans into a shared map frame and bootstraps coarse 3D bounding boxes from the aligned aerial detections; (ii) EM-like amodal box refinement alternates between identifying the vehicle's foreground points and updating the coarse box from the identified foreground and aerial priors. We evaluate DAA on an air-ground cooperative perception dataset, where it consistently outperforms existing auto-labeling baselines. LiDAR detectors trained on DAA-generated labels remain competitive with those trained on manual annotations at moderate IoU thresholds. Evaluation against a reference vehicle with known poses and dimensions further confirms its accuracy. We also demonstrate that DAA transfers, without parameter retuning, from onboard to stationary roadside LiDAR and to multi-agent fused point clouds. Code and dataset will be released upon publication.