  Dynamic neural radiance fields (dynamic NeRFs) have demonstrated impressive
results in novel view synthesis on 3D dynamic scenes. However, they often
require complete video sequences for training followed by novel view synthesis,
which is similar to playing back the recording of a dynamic 3D scene. In
contrast, we propose OD-NeRF to efficiently train and render dynamic NeRFs
on-the-fly which instead is capable of streaming the dynamic scene. When
training on-the-fly, the training frames become available sequentially and the
model is trained and rendered frame-by-frame. The key challenge of efficient
on-the-fly training is how to utilize the radiance field estimated from the
previous frames effectively. To tackle this challenge, we propose: 1) a NeRF
model conditioned on the multi-view projected colors to implicitly track
correspondence between the current and previous frames, and 2) a transition and
update algorithm that leverages the occupancy grid from the last frame to
sample efficiently at the current frame. Our algorithm can achieve an
interactive speed of 6FPS training and rendering on synthetic dynamic scenes
on-the-fly, and a significant speed-up compared to the state-of-the-art on
real-world dynamic scenes.
