Funny rotation differences between Quaternion wxyz and xyz Euler

Question

I am rigging my character. The Central thigh bone has copy rotation of FK thigh control bone. Pls refer the picture for clearer circumstances. I hit R -> Y -> -90 to open the leg to test rig. Then a puzzle ensues.

If I use Global orientation with Quaternion wxyz for rotation, the central thigh bone immediately misaligned. Any minor change to rotation any attribute will realign central thigh bone with FK thigh control bone.

If I use Global orientation with XYZ Euler for rotation, the rig works fine.

I also find out that the rig works fine using any other orientation (normal, local, etc.) with Global orientation.

If I grab the FK thigh control and rotate it manually, the rig also works fine.

Window console has not shown anything funny.

What gives?
Is my file broken? How can I fix it?

I would hate to redo all the works.

THX

Answer 1

Never treat XYZ of quaternions as XYZ of an Euler rotation. These are two completely different things, which only have the same identifiers.

If you use manually entered numerical values (angles) or animate, always use Euler. Internally, these are translated into quaternions.

Quaternions are also not really intended for the manual input of numerical values, but to calculate with them.

Furthermore, they are an essential part in the field of 3D, for example to solve the problem of the Gimbal Lock.

Answer 2

I'd stick to the default setting - Euler XYZ. There may be nothing wrong with the quaternion button. Quaternion numbers are 'complex numbers' - I don't follow the concept, but they're 4D (ergo the W next to XYZ). That's probably mucking up calculations and data that were amassed and made to function in a 3D space.