The K4 intrinsic parameter in nerfstudio doesn't match OpenCV or COLMAP

[devernay]

Describe the bug

OpenCV (source) and COLMAP (source) both use the rational fraction radial distortion model described In the OpenCV doc.

nerfstudio (source) uses a fourth order radial distortion polynomial, which has a totally different meaning for parameter k4. This seems to be inherited from other NeRF code.

Consequently, any project that uses a camera model with a non-zero k4 (which probably doesn't happen often) will be wrongly processed.

Several options:

• stop supporting k4 in nerfstudio
• stop supporting loading k4 in the colmap dataparser, and throw an error on non-zero k4, k5 or k6
• support COLMAP and OpenCV's k4 in nerfstudio, and throw an error on non-zero k5 or k6 in the COLMAP dataparser