error in initialization of quaternions about ukf HOT 4 CLOSED

@spaghetti- no worries – so this is actually intended, because of an inconsistency in the way Eigen represents quaternions. Despite the fact that the quaternion constructor Quaternion (const Scalar &w, const Scalar &x, const Scalar &y, const Scalar &z) which takes four scalar arguments takes them in the order (w, x, y, z), Eigen actually seems to represent quaternions internally with the components in the order (x, y, z, w), which is why the quaternion part of the state vector in the code you linked is defined that way. In any case, when initialising a quaternion from something like an Eigen::Matrix<double, 4, 1>, the elements are interpreted as being in the order (x, y, z, w). You can confirm this for yourself using the following:

Eigen::Quaterniond a = Eigen::Quaterniond(1.0, 0.0, 0.0, 0.0);
Eigen::Quaterniond b = Eigen::Quaterniond(Eigen::Matrix<real_t, 4, 1>(0.0, 0.0, 0.0, 1.0));
std::cout << a.w() << " " << a.vec().transpose() << std::endl;
std::cout << b.w() << " " << b.vec().transpose() << std::endl;

The output will be as follows:

1 0 0 0
1 0 0 0

Indicating that the way the two constructors have been used is equivalent.

from ukf.

@danieldyer I apologize for opening the issue before checking myself. In fact I did check but I only checked the case where I did Eigen::Quaterniond(1, 0, 0, 0). The Eigen guys did say that they aware of the inconsistency but are unable to do anything about it.

from ukf.

@danieldyer I also had a few questions regarding the calculation of the state vector derivate as you did here over equation 27 in A quaternion based unscented Kalman Filter by E. Kraft. Should I open a new issue to ask that or can I just email you if that is okay?

from ukf.

@spaghetti- Sure, no problem. My email address is visible on my profile now.

from ukf.