Make RealityCapture csv/ply exports work!

[jb-ye]

fix #3118

Changes in this PR:

1. fix cx,cy calculation which was done wrong previously
2. add --ply option to ns-process-data realitycapture to support splatfacto
3. safeguard brown4 tangential2 distortion model in full data manager, since opencv's undistort doesn't support it.

[kerrj]

lgtm!

[shengjie-lin]

@jb-ye You mentioned "safeguard brown4 tangential2 distortion model in full data manager, since opencv's undistort doesn't support it.". However, from my testing, changing k4 would affect the behavior of cv2.undistort. And sensors like Azure Kinect does provide k4 in the API. Can you please give some explanation on this? Thank you!

[jb-ye]

@shengjie-lin cv2.undistort takes k4, k5, k6 but they are used in the division model's denominator. The camera model supported by cv2.undistort is non-standard. Standard B4 model use k4 for the highest order polynomial term. In nerfstudio, we intend to support the standard Brown Tangential model, which is the case for nerfacto and other nerf models.

[shengjie-lin]

Thanks for the explanation @jb-ye . Does this mean COLMAP or hloc tool box also estimate the distortion coefficients in a non-compatible format from what opencv expects (specifically for the k4)?
Edit: sorry, I just checked and realized hloc only estimates k1, k2, p1, p2.

[jb-ye]

@shengjie-lin I am not sure about COLMAP, it support several cameras models. The FULL_OPENCV might be something you are looking for.