Colmap2nerf script transformation

[saimouli]

What does this set of transformation do?
https://github.com/NVlabs/instant-ngp/blob/master/scripts/colmap2nerf.py#L328

Why are y and z flipped and swapped?

Why is the quaternion flipped (https://github.com/NVlabs/instant-ngp/blob/master/scripts/colmap2nerf.py#L323)? Should the transform be wordl2cam and then inverted for cam2wld transform?

What convention should the cam2wld be? Documentation on transformations and conventions used would be helpful

[Vadkoz]

I just passing by, but anyway, maybe this discussion will be helpful.

Anyway, mentioned quaternion flipping is just a code mistake because the rotation of Q is the same as the rotation of -Q due to math reasons. So maybe it's not a mistake, but rather a redundancy.

Y and Z flipped, probably because of convention. You can project as like as on this image, but also you can do it in other direction: to the rigtht, so order of points on line become O_c, P_c, P. The for change convention you need flip Y and Z axes. I can be wrong, I really don't remember what did i do here a year ago :] Math can be tricky here. For sure documentation is highly required for such projects.

(i took picture from this book, page 55)

upd:
Indeed:

Pretty usefull book. Recommend ;)