quaternions_to_euler.py

import bpy

def get_or_create_fcurve(action, data_path, array_index=-1, group=None):
	for fc in action.fcurves:
		if fc.data_path == data_path and (array_index<0 or fc.array_index == array_index):
			return fc

	fc = action.fcurves.new(data_path, array_index)
	fc.group = group
	return fc

def add_keyframe_euler(action, euler, frame, bone_prefix, group):
	for i in range(len(euler)):
		fc = get_or_create_fcurve(action, bone_prefix+"rotation_euler", i, group)
		pos = len(fc.keyframe_points)
		fc.keyframe_points.add(1)
		fc.keyframe_points[pos].co = [frame, euler[i]]
		fc.update()


def frames_matching(action, data_path):
	frames = set()
	for fc in action.fcurves:
		if fc.data_path == data_path:
			fri = [kp.co[0] for kp in fc.keyframe_points]
			frames.update(fri)
	return frames


def fcurves_group(action, data_path):
	for fc in action.fcurves:
		if fc.data_path == data_path and fc.group is not None:
			return fc.group
	return None


def convert_quaternion_to_euler(action, obj, order):

	bone_prefixes = set()
	for fc in action.fcurves:
		if fc.data_path == "rotation_quaternion" or fc.data_path[-20:]==".rotation_quaternion":
			bone_prefixes.add(fc.data_path[:-19])
	
	print(bone_prefixes)


	for bone_prefix in bone_prefixes:
		if (bone_prefix == ""):
			bone = obj
		else:
			bone = eval("obj."+bone_prefix[:-1]) # I wish I knew a better way to do this

		data_path = bone_prefix + "rotation_quaternion"
		frames = frames_matching(action, data_path)
		group = fcurves_group(action, data_path)

		for fr in frames:
			quat = bone.rotation_quaternion.copy()
			for fc in action.fcurves:
				if fc.data_path == data_path:
					quat[fc.array_index] = fc.evaluate(fr)
			euler = quat.to_euler(order)

			add_keyframe_euler(action, euler, fr, bone_prefix, group)
			bone.rotation_mode = order
#
#

scn = bpy.context.scene
obj = bpy.context.active_object
action = obj.animation_data.action
order='XYZ'

convert_quaternion_to_euler(action, obj, order)