setFromAxisAngle( axis, angle ) {

		// http://www.euclideanspace.com/maths/geometry/rotations/conversions/angleToQuaternion/index.htm

		const halfAngle = angle / 2, s = Math.sin( halfAngle );

		this._x = axis.x * s;
		this._y = axis.y * s;
		this._z = axis.z * s;
		this._w = Math.cos( halfAngle );

		this._onChangeCallback();

		return this;

	}
Sets this quaternion from the given axis and angle.

@param {Vector3} axis - The normalized axis.
@param {number} angle - The angle in radians.
@return {Quaternion} A reference to this quaternion.