In our voxl-camera-server, we have a default intrinsics config for the IMX412 camera / lens as follows (focal length approximation is based on a several calibrations). Basically it is a fisheye lens with about 2000px focal length at full resolution (half of that at half resolution, etc). The camera full sensor width is 4056 (hence the x principal point is exactly half of that 2028). In some cases we use slightly trimmed frame (4040 in width). But in any case, that should be enough for a good starting point.
lens_cal.width = 4040; lens_cal.height = 3040; lens_cal.fx = 1999.0; lens_cal.fy = 1999.0; lens_cal.cx = 2028.0; lens_cal.cy = 1520.0; lens_cal.is_fisheye = 1;Alex