How to include pitch and roll in the registration constraints

[LanWu076]

Such an awesome project ! Thank you for sharing the code!
I have a question about the optimization. How to add Pitch and Roll terms in the registration constraints if we don't have the IMU to provide observable gravity?
Pitch and Roll will change the second term of the Jacobian. Will it also affect the first term, the trilinear interpolation function?
Looking forward to your reply :)