I want to rotate an object towards mouse position on screen in 3d environment. How can I do this?

Questions & Answers
1

I mean where the mouse or cursor position is, object should be facing towards all the time.

2

Here is a simple solution. It works but has it downsides.

	Camera cam;

	// Use this for initialization
	void Start () {
		cam = Camera.main;
	}
	
	// Update is called once per frame
	void Update () {
		Ray ray = cam.ScreenPointToRay(Input.mousePosition);;
		RaycastHit hit;

		if (Physics.Raycast(ray, out hit)) {

			Vector3 target = hit.point;
			target.y = 0;
			target.y = transform.localScale.y / 2f;

			transform.LookAt (target);
		}
	}
4

Here’s a fairly short and (IMO) sleek C# class file (a MonoBehaviour component) you can use for this kind of problem.

using UnityEngine;

public class MoveToMouse : MonoBehaviour
{
    //  e.g. 0.2 = slow, 0.8 = fast, 2 = very fast, Infinity = instant
    [Tooltip("If rotationSpeed == 0.5, then it takes 2 seconds to spin 180 degrees")]
    [SerializeField] [Range(0,10)] float rotationSpeed = 0.5f;

  
    [Tooltip("If isInstant == true, then rotationalSpeed == Infinity")]
    [SerializeField] bool isInstant = false;
  
    Camera _camera = null;  // cached because Camera.main is slow, so we only call it once.

    void Start()
    {
        _camera = Camera.main;
    }
  
 
    void Update()
    {
        Ray ray = _camera.ScreenPointToRay(Input.mousePosition);
        Quaternion targetRotation = Quaternion.LookRotation(ray.direction);
   
        if (isInstant)
        {
            transform.rotation = targetRotation;
        }
        else
        {
            Quaternion currentRotation = transform.rotation;
  
            float angularDifference = Quaternion.Angle(currentRotation, targetRotation);  
            // will always be positive (or zero)
  
            if (angularDifference > 0) transform.rotation = Quaternion.Slerp(
                                         currentRotation,
                                         targetRotation,
                                         (rotationSpeed * 180 * Time.deltaTime) / angularDifference
                                    );
            else transform.rotation = targetRotation;
        }
    }
}

Seems to work very well for me and seems to answer the question, so anyone who comes here can use/adapt/hack my code :slight_smile: Let me know if I made any mistakes or you have some suggestions to improve, although it works super smoothly and intuitively in my star-fox style rail shooter game I’m making with Ben Tristem’s GameDev_dot_TV course on Udemy (“C# Unity Developer 3D”).

3

using UnityEngine;
public class Example : MonoBehaviour
{
//assign this field in inspector:
public Transform subject;
public float _depthOffset = 0f;

    Camera _camera;

    void Start ()
    {
        //copy a reference to main camera, for both convenience and performance:
        _camera = Camera.main;
    }

    void Update ()
    {
        //update rotation:
        Vector3 mousePositionWorldSpace = _camera.ScreenToWorldPoint( Input.mousePosition );
        Vector3 pointOfInterest = mousePositionWorldSpace+_camera.transform.forward*_depthOffset;
        Vector3 direction = pointOfInterest-subject.position;
        subject.rotation = Quaternion.LookRotation( direction );
    }
}
5

I coded it so that the rotating object looks at the mouse even with or without a raycast hit. This should work. I basically combined the answers from @GregoryFenn and @Cornelis-de-Jager

public Camera cam;
public Transform obj;
public float rotationSpeed;

void Update()
{
    Quaternion currentRotation = obj.rotation;
    Quaternion targetRotation;

    Ray ray = cam.ScreenPointToRay(Input.mousePosition);
    RaycastHit hit;

    if (Physics.Raycast(ray, out hit))
    {
        targetRotation = Quaternion.LookRotation(hit.point - obj.position);
    }
    else
    {
        targetRotation = Quaternion.LookRotation(ray.direction);
    }
    float angularDifference = Quaternion.Angle(currentRotation, targetRotation);

    if (angularDifference > 0)
    {
        obj.rotation = Quaternion.Slerp
        (
            obj.rotation,
            targetRotation,
            rotationSpeed * Time.deltaTime
        );
    }
    else
    {
        obj.rotation = targetRotation;
    }
}