DynamicCity: Large-Scale LiDAR Generation from Dynamic Scenes
Authors: name: Hengwei Bian, name: Lingdong Kong, name: Haozhe Xie, name: Liang Pan, name: Yu Qiao, name: Ziwei Liu

LiDAR scene generation has been developing rapidly recently. However,
existing methods primarily focus on generating static and single-frame scenes,
overlooking the inherently dynamic nature of real-world driving environments.
In this work, we introduce DynamicCity, a novel 4D LiDAR generation framework
capable of generating large-scale, high-quality LiDAR scenes that capture the
temporal evolution of dynamic environments. DynamicCity mainly consists of two
key models. 1) A VAE model for learning HexPlane as the compact 4D
representation. Instead of using naive averaging operations, DynamicCity
employs a novel Projection Module to effectively compress 4D LiDAR features
into six 2D feature maps for HexPlane construction, which significantly
enhances HexPlane fitting quality (up to 12.56 mIoU gain). Furthermore, we
utilize an Expansion & Squeeze Strategy to reconstruct 3D feature volumes in
parallel, which improves both network training efficiency and reconstruction
accuracy than naively querying each 3D point (up to 7.05 mIoU gain, 2.06x
training speedup, and 70.84% memory reduction). 2) A DiT-based diffusion model
for HexPlane generation. To make HexPlane feasible for DiT generation, a Padded
Rollout Operation is proposed to reorganize all six feature planes of the
HexPlane as a squared 2D feature map. In particular, various conditions could
be introduced in the diffusion or sampling process, supporting versatile 4D
generation applications, such as trajectory- and command-driven generation,
inpainting, and layout-conditioned generation. Extensive experiments on the
CarlaSC and Waymo datasets demonstrate that DynamicCity significantly
outperforms existing state-of-the-art 4D LiDAR generation methods across
multiple metrics. The code will be released to facilitate future research.