Multiply the distance by the norm of direction

[dichen-cd]

Hi there, thanks for sharing the code!

When computing the distances of samples during rendering, some open sourced codes such as nerf_pl and mip-nerf multiplies the distances with the norms of direction. Seems like there's no such operation in this repo. (volume_op.py#L191) Could you give any hint on the difference?

[ziruiw-dev]

Hi @DeanChan,

Sorry for the late reply.

Good catch, we do not have this multiplication in our code.

If you print the output of torch.norm(ray_dir_world, dim=-1), you can see that the magnitudes of direction vectors are all very close to 2.0. So we omit this multiplication, which in this case is just multiplying a constant with the dists and has very little effect on training.

These magnitudes ~ 2.0 is the result of using NDC, so this omission is fine in forward-facing scenes.

If you switch off NDC by setting --use_ndc=False, you can see that the variation of the magnitudes of all direction vectors is slightly larger, i.e. from 1.0 to 1.3 or so, depending on the camera FOV. That being said, if you want to apply it to non-forward-facing scenes, you probably want to add this multiplication (dist = dist * norm) back. However, we do not observe a big difference with or without this multiplication even in non-forward-facing scenes.

Best,
Zirui

[dichen-cd]

Hi @ziruiw-dev , thank you for your reply. It is really helpful!

I've got similar findings through my own scenes that multiply the dist norm or not does not show any significant difference.