2D Rotate towards GameObject acting odd.

Questions & Answers
1

Hey, I am having a problem with a 2d (air)plane I am working on. It is a side scrolling game, and I want the plane to follow the player. To move I am using the rigidbody2D and to rotate I am using Quaternion LookAt. The problem is, when the aircraft exceeds the X position of the player, it goes in the opposite direction, so it’s moving away from the player instead of towards it. When it gets behind the player again ( X<=player X), it follows the player normally.

void FixedUpdate () 
{
    	Rotate ();
    	Move ();
}
    
void Rotate()
{	
	Quaternion newRotation = Quaternion.LookRotation (player.transform.position - transform.position);
	Quaternion newRot = Quaternion.Slerp(transform.rotation, newRotation, rotateSpeed * Time.deltaTime);
	transform.rotation = new Quaternion(0, 0, newRot.z, newRot.w);
}
    
void Move()
{
	rigidbody2D.AddForce (transform.right * flySpeed*8 * Time.deltaTime, ForceMode2D.Impulse);
    
	if(rigidbody2D.velocity.magnitude > flySpeed)
	{
		rigidbody2D.velocity = rigidbody2D.velocity.normalized * flySpeed;
	}
}

I want the aircraft to rotate with a rotatespeed, so it won’t instantly face the player, and has to make a nice loop when it needs to.
Can anyone help me with this? I’ve looked into multiple solutions, but haven’t found anything.

3

You should perform your calculations in local space not in world space.

You can rewrite your Rotate() method like this:

    void Rotate()
    {
        Vector2 dir = transform.InverseTransformPoint(player.transform.position);

        float angle = Vector2.Angle(Vector2.right, dir);

        angle = dir.y < 0 ? -angle : angle;

        transform.Rotate(Vector3.forward, rotateSpeed * Time.deltaTime * angle);
    }

This way you have a very simple but solid implementation of a follow script.

To prevent the follower to increase its angular velocity with the size of the angle you could
do the following:

    const float Threshold = 1e-3f;
    void Rotate()
    {
        Vector2 dir = transform.InverseTransformPoint(player.transform.position);

        float angle = Vector2.Angle(Vector2.right, dir);

        angle = dir.y < 0 ? -angle : angle;

        if (Mathf.Abs(angle) > Threshold)
        {
            transform.Rotate(Vector3.forward, rotateSpeed * Time.deltaTime * Mathf.Sign(angle));
        }
    }
2

Okay, so this is a little ‘hacky’ in that I create a new parent so that I can separate the rotation around z and the local rotation around x (to make the plane upright). There would be a way of doing this without the extra parent, but I should be wrapping presents and not spending hours on Quaternion rotations! :smiley:

The following should be attached to the highest object in your plane object hierarchy:

public float forwardSpeed = 5;
public float turnSpeed = 90;
public float twistSpeed = 180;
public Transform target;
Vector3 targetPos;
Transform this_transform;
Vector3 dir;
Transform parentT;
GameObject parentGO;

void Start(){
	this_transform = transform;
	parentGO = new GameObject("rotationControl");
	parentT = parentGO.transform;
	parentT.position = this_transform.position;
	this_transform.parent = parentT;
}

void FixedUpdate(){
	dir = new Vector3(target.position.x - parentT.position.x, target.position.y - parentT.position.y, 0);
	
	//Look rotation (around Z axis)
	Quaternion q = Quaternion.FromToRotation(Vector3.right, dir);
	parentT.rotation = Quaternion.RotateTowards(parentT.rotation, q, turnSpeed*Time.deltaTime);
	
	//Twist rotation (around local X axis)
	Quaternion q2 = Quaternion.identity;
	if(Mathf.Sign(dir.x) < 0){
		q2 = Quaternion.Euler(180, 0, 0);
	}
	this_transform.localRotation = Quaternion.RotateTowards(this_transform.localRotation, q2, twistSpeed*Time.deltaTime);
	
	//Movement (you can use your forces here, this was mainly for testing)
	parentT.position += parentT.right*Time.deltaTime*forwardSpeed;
}

This should turn the plane to face the target over time and upright the plane when it becomes upside-down, hopefully without causing any movement in the z axis (you may want to clamp the z value just to be safe)!

Merry Christmas/Happy Holidays!

Scribe