Apply rotations, first about world up, then about (travelling) local right. rot.y is rotation about worldUp, and rot.x is second rotation, about local right.

Get the rotations, first about world up, then about (travelling) local right, necessary to align the quaternion's forward with the target direction. This represents the tripod head movement needed to look at the target. This formulation makes it easy to interpolate without introducing spurious roll.

Normalize a quaternion

This is a slerp that mimics a camera operator's movement in that it chooses a path that avoids the lower hemisphere, as defined by the up param