quaternions melted my brain years ago.

They are required to get the world transforms for body parts as I understand it.

One of the python gurus has some code to do the calculations. Try posting a specific request to this forum.  I only did maths 40 years ago and I dont remember much about array calculations. ;(

ATAN2 seems to aavoid the need for a buch of IF statements to find what quadrant you are in to add the correction to the atan derived angle.

 

My motto for code is "whatever works ! "