Interesting. I was implementing this function in my project a couple months ago. The half-angle identities seemed promising and I wondered if an optimization li

Interesting. I was implementing this function in my project a couple months ago. The half-angle identities seemed promising and I wondered if an optimization like this were possible. Alas, I also needed the angle to compute the torque applied by an angular spring, so I slapped an arccos in there and called it a day.