In unity3d, billboard effect can be realized towards another target

In yesterday's work , I want to realize the special effect of meteors crossing the sky . The face normal faces the camera and the head faces the flying target .

The idea is , It is realized by pasting a picture on the model piece and moving it . Just use the billboard . I thought it was a very simple thing ！ The result is lookat function , Realization mesh Facing the camera is OK , But the head of a meteor can never face the direction of the flying target . Always fly sideways and sideways . A very simple thing took a lot of brains to come up with .

How on earth can we make the two axes of an object face different objects , It seems that only lookat No good , You still have to write your own code .

This effect is achieved in two steps ：

1. Make an axis of the object face the target point .

2. Make the other axis of the object face the camera .（ This step should not affect the previous step x Axis orientation ）

About the first step , I refer to the content of this blog

https://blog.csdn.net/qq_42541751/article/details/114987971?share_token=553A6EAB-54D8-46EC-82CA-018FBB4215CE&tt_from=weixin&utm_source=weixin&utm_medium=toutiao_ios&utm_campaign=client_share&wxshare_count=1

Code up ：

        //  toward target vector 
        var targetDir = looktarget.position - gameObject.transform.position;
        //  Specify which axis faces the target  
        var fromDir = gameObject.transform.right;// gameObject.transform.rotation * gameObject.transform.right;
        // Calculate the vector perpendicular to the orientation axis and the target direction （ Rotation axis ）
        var axis = Vector3.Cross(fromDir, targetDir).normalized;
        // Calculate the angle between the current direction and the target direction 
        var angle = Vector3.Angle(fromDir, targetDir);
        // Rotate the current direction to the target direction by a certain angle , This angle value can be interpolated 
        gameObject.transform.rotation = Quaternion.AngleAxis(angle, axis) * gameObject.transform.rotation;

About the second step

First , Calculate the camera vector at YOZ Projection in plane , Then find the projection vector and y Angle between axes , And then according to x Axis to rotate this

Angle is OK .

        var camdir = cam.transform.position - gameObject.transform.position;
        var touying = ProjectOnPlane(camdir, gameObject.transform.right);
        var q = Quaternion.FromToRotation(gameObject.transform.up, touying);
        gameObject.transform.rotation = q * gameObject.transform.rotation;

    //  Find the projection vector of the vector on the plane 
    Vector3 ProjectOnPlane(Vector3 vp, Vector3 vn)
    {
        Vector3 vt = new Vector3();
        vt = vp - vn * Vector3.Dot(vp, vn) / Vector3.Dot(vn, vn);
        return vt;
    }

Final effect ：