  Online reconstructing and rendering of large-scale indoor scenes is a
long-standing challenge. SLAM-based methods can reconstruct 3D scene geometry
progressively in real time but can not render photorealistic results. While
NeRF-based methods produce promising novel view synthesis results, their long
offline optimization time and lack of geometric constraints pose challenges to
efficiently handling online input. Inspired by the complementary advantages of
classical 3D reconstruction and NeRF, we thus investigate marrying explicit
geometric representation with NeRF rendering to achieve efficient online
reconstruction and high-quality rendering. We introduce SurfelNeRF, a variant
of neural radiance field which employs a flexible and scalable neural surfel
representation to store geometric attributes and extracted appearance features
from input images. We further extend the conventional surfel-based fusion
scheme to progressively integrate incoming input frames into the reconstructed
global neural scene representation. In addition, we propose a highly-efficient
differentiable rasterization scheme for rendering neural surfel radiance
fields, which helps SurfelNeRF achieve $10\times$ speedups in training and
inference time, respectively. Experimental results show that our method
achieves the state-of-the-art 23.82 PSNR and 29.58 PSNR on ScanNet in
feedforward inference and per-scene optimization settings, respectively.
