Quaternion value imported from VMD file is incorrect at some point about blender_mmd_tools HOT 1 OPEN

I did some debugging, my finding is that: in the vmd/importer.py

For the BoneConverter.convert_rotation:

rot = Quaternion()
rot.x, rot.y, rot.z, rot.w = rotation_xyzw
return Quaternion(matmul(self.__mat, rot.axis) * -1, rot.angle).normalized()

I don't quite understand why we need to do that matmul calucation there. But if we return rot directly, the bone rotation of my above example would become visually correct. However, other bones would go wrong. So I guess, in some cases, we don't need to do that calcuation?

I also noticed that the error bone rotation typically happens when the rotation value change dramatically: like in the following diragram, it happen at those troughs.

from blender_mmd_tools.