  No significant work has been done to directly merge two partially overlapping
scenes using NeRF representations. Given pre-trained NeRF models of a 3D scene
with partial overlapping, this paper aligns them with a rigid transform, by
generalizing the traditional registration pipeline, that is, key point
detection and point set registration, to operate on 3D density fields. To
describe corner points as key points in 3D, we propose to use universal
pre-trained descriptor-generating neural networks that can be trained and
tested on different scenes. We perform experiments to demonstrate that the
descriptor networks can be conveniently trained using a contrastive learning
strategy. We demonstrate that our method, as a global approach, can effectively
register NeRF models, thus making possible future large-scale NeRF construction
by registering its smaller and overlapping NeRFs captured individually.
