How to make a true linear quaternion rotation? It turns out to be impossible to make a 360° rotation by using quaternion only with 2 keyframes. So I used three. However, the result appears unlinear, even though the curve interpolation type are all set to Linear. demo file

I wonder if this is something unavoidable. If not, is there any way to achieve a true linear quaternion rotation?
You would need to make a sinus/cosinus graph for that animation to have linear motion with quaternions:

For the 1st cube I set Euler rotation and animated a linear rotation, with Copy rotation constraint copied that to the second cube and then baked the animation on it with Object > Animation > Bake Action (visual keying on). This resulted in (the gif is little laggy):



This could be done easily with Sinusoidal easing type. Setting correct easingIn/easingOut for keyframes yields to this nice result: