Apply relative rotation from one armature to another

Coding Python Support
1

I have an armature in pose mode on which I am able to rotate the bones around and I want to replicate the relative rotation of each bone on another armature. So if I rotate the arm by 90 degree on the first armature, the same should happen on the other one.

Here is the code I use for every bone in the target armature:

# Calculate rotation on the reference armature to get from the its tpose to its new pose
offset_to_new_pose = reference_tpose.inverted() @ reference_new_pose

# Add this rotation to the target tpose
bone.rotation_quaternion = target_tpose @ offset_to_new_pose

Here is the reference armature in tpose and in new pose:
reference armature: tpose and target pose

And here is the target armature in tpose and in the calculated new pose:
enter image description here

As you can see the bones are rotated in a different direction compared to the reference armature. But I want to replicate the rotation in the same direction.

Here is the exact calculation results for the selected bone in the images:

# Calculation of the offset from the reference armature
reference_tpose = Quaternion (w=0.7071, x=-0.0000, y=0.7071, z=-0.0000)
reference_new_pose = Quaternion (w=-0.4945, x=-0.4722, y=-0.5239, z=0.5080)
offset_to_new_pose = Quaternion (w=-0.7201, x=-0.6931, y=-0.0208, z=0.0253)

# Bone rotation of target armature before adding offset
bone.rotation_quaternion = Quaternion (w=1.0000, x=0.0000, y=0.0000, z=0.0000)

# Bone rotation after adding offset
bone.rotation_quaternion = Quaternion (w=-0.7201, x=-0.6931, y=-0.0208, z=0.0253)

What am I missing here? Is my calculation incorrect? Do I have to account for something that I’m currently missing? Are the axes incorrect?

Thanks in advance!

2

If you want relative rotation, you can’t assign absolute values to the target bone. It needs to be culmulative.

Try something like this

offset_to_new_pose = reference_new_pose.inverted() @ reference_tpose

# Target
bone.rotation_quaternion @= offset_to_new_pose.inverted()
3

Hey thanks for your answer!
That solution didn’t work. I have inherit_rotation turned off for each bone, so I think that there is no difference between adding the offset to the current bone rotation or setting it as a new rotation calculated from the tpose.