unity lookat实现原理是什么？

Question

unity 官方文档里对lookAt的说明:
Rotates the transform so the forward vector points at /target/'s current position.

Then it rotates the transform to point its up direction vector in the direction hinted at by the worldUp vector. If you leave out the worldUp parameter, the function will use the world y axis. worldUp is only a hint vector. The up vector of the rotation will only match the worldUp vector if the forward direction is perpendicular to worldUp.

这里第二段说在第二次旋转后，使得当前对象的正y轴与worldUp方向匹配，并不是完全相同。
想知道这里第二次旋转后y轴最终的具体方向究竟是如何运算的？
在网上查了，下面这篇也没看明白。
https://blog.csdn.net/kfqcome...

经过多次测试后发现，worldUp的描述“在完成上面的旋转之后，继续旋转自身，使得当前对象的正y轴朝向与worldUp所指向的朝向一致”里的朝向一致，指的是新旋转后的y轴与worldUp在该对象初次旋转后的xy平面上的投影向量一致。也就是说，worldUp应取它在应用了旋转量q后的xy平面的投影量。

using UnityEngine;
using System.Collections;
 
public class lookAtTest : MonoBehaviour
{
 
    public Transform target;
    void Start()
    {
        target.transform.position = Vector3.zero;
        transform.position = new Vector3(0, 30, 60);
 
        Vector3 diff = target.position - transform.position;
        Quaternion q = Quaternion.FromToRotation(Vector3.forward, diff);
        
        Vector3 n = q * Vector3.forward;
        Vector3 worldUp = Vector3.up;
        float dirDot = Vector3.Dot(n, worldUp);
        Vector3 vProj = worldUp - n * dirDot;    //worldUp在xy平面上的投影量
        vProj.Normalize();
 
    
        float dotproj = Vector3.Dot(vProj, newUp);
        float theta = Mathf.Acos(dotproj) * Mathf.Rad2Deg;
        Quaternion qNew = Quaternion.AngleAxis(theta, n);        
 
        Quaternion qall = qNew * q;
        transform.rotation = qall;
    }
}

上面代码没看懂啊。