How to calculate rotation just like in MotionBuilder

Question

Problem:

My goal is to write a code, that rotates the root joint of a bvh, θ degrees around global y axis3, and keeps values in the range of -180 to 180 (just like MotionBuilder does). I have tried to rotate a joint using euler, quaternions, matrices (considering the rotational order of a bvh) but I haven't yet figured out how to get the correct values. MotionBuilder calculates the values x,y,z so they are valid for the bvh file. I would like to write a code that calculates the rotation x,y,z for a joint, just like in MotionBuilder.

Example:

Initial: Root rotation: [x= -169.56, y=15.97, z=39.57]

After manually rotating about 45 degrees: Root rotation: [x=-117.81, y=49.37, z=70.15]

global y axis:

Answer 1

To rotate a node around the world Y axis any number of degrees the following works (https://en.wikipedia.org/wiki/Rotation_matrix):

import math
from pyfbsdk import *

angle = 45.0
radians = math.radians(angle)
root_matrix = FBMatrix()
root.GetMatrix(root_matrix, FBModelTransformationType.kModelRotation, True)

transformation_matrix = FBMatrix([
    math.cos(radians), 0.0, math.sin(radians), 0.0,
    0.0, 1.0, 0.0, 0.0,
    -math.sin(radians), 0.0, math.cos(radians), 0.0,
    0.0, 0.0, 0.0, 1.0
])

result_matrix = root_matrix * transformation_matrix
root.SetMatrix(result_matrix , FBModelTransformationType.kModelRotation, True)

If there are any Pre-Rotations on the root node the process is more complex and you can try setting the Rotations using the SetVector with the LRMToDof method.

result_vector = FBVector3d()
root.LRMToDof(result_vector, result_matrix)
root.SetVector(result_vector, FBModelTransformationType.kModelRotation, True)