1.物体以中心点建立轴，绕轴旋转

    /// <summary>
    /// 将物体绕轴旋转，且始终朝向中心
    /// </summary>
    /// <param name="obj">旋转物体</param>
    /// <param name="center">旋转中心</param>
    /// <param name="axis">旋转轴</param>
    /// <param name="angle">绕轴旋转角度</param>
    public void MyRotateAround(Transform obj, Vector3 center, Vector3 axis, float angle)
    {

        Vector3 pos = obj.position; //要旋转物体的坐标

        Quaternion rot = Quaternion.AngleAxis(angle, axis); //按照axis向量为轴，旋转angle角度的四元数

        Vector3 dir = pos - center; //从圆心指向摄像头的朝向向量

        dir = rot * dir;            //旋转此向量（看来是这个意思，旋转后的四元数*旋转前朝向向量，即为最终朝向向量）

        obj.position = center + dir;//移动摄像机位置

        Quaternion myrot = obj.rotation; //旋转前的四元数

        //transform.rotation *= Quaternion.Inverse(myrot) * rot *myrot;//设置角度另一种方法

        obj.rotation = rot * myrot; //设置角度（看来是这个意思，旋转后的四元数*旋转前的四元数，即为最终朝向四元数）

    }

2.物体以中心点默认的世界坐标轴x,y,z轴，三个轴同时旋转（注释同上）

    public void MyRotateAround(Transform obj, Vector3 center, float xAngle = 0, float yAngle = 0, float zAngle = 0)
    {
        Vector3 pos = obj.position;

        Quaternion rot = Quaternion.Euler(xAngle, yAngle, zAngle);

        Vector3 offset = pos - center;

        offset = rot * offset;

        obj.position = center + offset;

        Quaternion myrot = obj.rotation;

        obj.rotation = rot * myrot;

    }

1.Quaternion.AngleAxis(angle,axis);创建一个四元数，绕axis轴旋转angle角度
2.Quaternion.Euler(mX, mY, mZ);创建一个四元数，绕x轴旋转mX角度，绕y轴旋转mY角度，绕z轴旋转mZ角度
3.旋转矢量：旋转后的四元数*旋转前朝向向量，即为最终朝向向量

image.png

4.四元数叠加：四元数相乘，是四元数的叠加;pxq，是在q旋转量的基础上，叠加p

例子：
鼠标右键旋转相机，观测物体

    if (Input.GetMouseButton(1))
    {
        mouseX = Input.GetAxis("Mouse X") * Time.deltaTime * leftrightRotateSpeed;  //绕y轴旋转角度
        mouseY = Input.GetAxis("Mouse Y") * Time.deltaTime * updownRotateSpeed;  //绕x轴旋转角度

        direc1 = camTrans.position - playerTrans.position;  //中心点到相机向量
        direc2 = new Vector3(direc1.x, direc1.y - 10, direc1.z);    //中心点到相机下方10位置的向量
        direc = Vector3.Cross(direc1, direc2);  //上面两个向量确定了上下方向的旋转轴向量

        Common.Instance.MyRotateAround(camTrans, playerTrans.position, mouseY * direc.x, mouseX, mouseY * direc.z);

    }

例2：简易freecamera

    using System.Collections;
    using System.Collections.Generic;
    using UnityEngine;

    public class FreeCamera : MonoBehaviour {

        Transform CamTrans;

        [Range(0.1f,20)]
        public float MoveSpeed = 1;
        [Range(0.1f, 20)]
        public float RotateSpeed = 1;

        protected Vector2 m_Rotation;

        private void Start()
        {
            CamTrans = GetComponent<Transform>();
        }

        private void Update()
        {
            CamTrans.position += (CamTrans.forward * Input.GetAxis("Vertical") + CamTrans.right * Input.GetAxis("Horizontal") + CamTrans.up * Input.GetAxis("Updown")) * MoveSpeed;

            float UpAngle = Input.GetAxis("Mouse X");
            float RightAngle = -Input.GetAxis("Mouse Y");
            float XRightSeparateAngle = RightAngle * Vector3.Dot(CamTrans.right, Vector3.right);
            float ZRightSeparateAngle = RightAngle * Vector3.Dot(CamTrans.right, Vector3.forward);
            Quaternion rot = Quaternion.Euler(XRightSeparateAngle * RotateSpeed, UpAngle * RotateSpeed, ZRightSeparateAngle * RotateSpeed);

            CamTrans.rotation =  rot * CamTrans.rotation;

        }
    }

例3：物体绕自身轴旋转

public float RotateSpeed;
public rotateAxis RotateAxis = rotateAxis.Z;
public enum rotateAxis
{
    X, Y, Z
}

Vector3 mousePos;

private void Update()
{
    if(Input.GetKey(KeyCode.Mouse0))
    {

        float xDistance = Input.mousePosition.x - mousePos.x;
        //float yDistance = Input.mousePosition.y - mousePos.y;


        Vector3 rotAxis = Vector3.zero;
        switch(RotateAxis)
        {
            case rotateAxis.X:
                rotAxis = transform.right;
                break;

            case rotateAxis.Y:
                rotAxis = transform.up;
                break;

            case rotateAxis.Z:
                rotAxis = transform.forward;
                break;
        }

        MyRotateAround(transform, transform.position, rotAxis, xDistance * RotateSpeed);

        mousePos = Input.mousePosition;
    }
}