Python, How to get pose bone to rotate with another pose bone from a different armature? Finding Rotational difference

Question

I have two Armatures with different bone structure.

The goal is to get the rotational difference of one bone from a one Armature and translate it to the bone of the other Armature.

Code **For a General approach:

import bpy

context = bpy.context
scene = context.scene

Rig_1 = bpy.context.scene.objects['__Rig'] 
Rig_2 = bpy.context.scene.objects['__Rig2'] 

Rig_1_RightArm = Rig_1.pose.bones["r_uparm"]
Rig_2_RightArm = Rig_2.pose.bones["r_uparm"]

scene.frame_set(1)
WorldSpace_RightArm_1-1 = Rig1.matrix_world @ Rig_1_RightArm.matrix 

bpy.context.view_layer.update()

scene.frame_set(2)
WorldSpace_RightArm_1-2 = Rig1.matrix_world @ Rig_1_RightArm.matrix 
WorldSpace_RightArm_2-2 = Rig2.matrix_world @ Rig_2_RightArm.matrix 

bpy.context.view_layer.update()

I've tried multiple methods and would like to know what to do beyond here, like using rotation_difference to between WorldSpace_RightArm_1-1 from frame 1 and WorldSpace_RightArm_1-2 from frame 2.

Answer 1

Object.convert_space

As mentioned in previous answer there is a convenience method available on the object that converts space.

Rather than totally doing my head in by trying to make a rotation difference am going to

• Convert source and target pose bone matrices to world space
• Decompose them, ie get their world space location, rotation and scale parts.
• Recompose the target world matrix to have rotation part of source
• Convert this to pose space of target bone and set as matrix.

Test method.

import bpy
from mathutils import Matrix

def snap_rotation(source, target):
    Ms = source.id_data.convert_space(
        pose_bone=source,
        matrix=source.matrix,
        from_space='POSE',
        )
    Mt = target.id_data.convert_space(
        pose_bone=target,
        matrix=target.matrix,
        from_space='POSE',
        ) 
         
    _, q, _ = Ms.decompose()
    t, _, s = Mt.decompose()
    M = (
        Matrix.Translation(t) @
        q.to_matrix().to_4x4() @ 
        Matrix.Diagonal(s.to_4d())
        )
    target.matrix = target.id_data.convert_space(
        pose_bone=target,
        matrix=M,
        to_space='POSE',
        )    

if __name__ == "__main__":
    context = bpy.context
    scene = context.scene
    snap_rotation(
        scene.objects["Armature"].pose.bones["Bone"],
        scene.objects["Armature.001"].pose.bones["Bone"]
    )