Mathf.Atan changes instantly even with a slow lerped value

Question

I have two scripts. One is for the players x position and the other one is for the players y rotation. The position script acts like a one dimensional joystick, that goes to its original position over time. Im using Mathf.Atan(), to rotate the player based on the x axis of my "joystick". I used lerp, so the joystick goes slowly into his original position. With the lerp changed, so changes its rotation. But there is a problem. I have a currentPos (the updating mouse position) and a StartPos2 (the mouse position that sets if you click). The StartPos2 goes slowly to the position of currentPos (the distance will be 0 then). The rotation somhow changes instantly, but it should change slowly because of the lerp.

Rotation script (if i remove the * 20, value will be lower but problem doesnt go away):

 Quaternion GetQuaternion()
     {
         Quaternion rot;
         
         float angle;
 
         angle = Mathf.Atan(GetLerpedMoveDist() * Mathf.Rad2Deg / 2) * 20;
         
         rot = Quaternion.Euler(0, -angle, 0);
         
         return rot;
     }

"Joystick" script:

 float GetLerpedMoveDist()
     {
         float lerpDist = startPos2.x - currentPos.x;
         startPos2 = Vector2.Lerp(startPos2, currentPos, Time.deltaTime * rotSmooth);
         return lerpDist;
     }

Answer 1

startPos2.x - currentPos.x; is not a valid input for arc tangens ( y/x is ).
To smoothly rotate something - you need to interpolate between angles or quaternions, not position vectors.
You may want to use atan2 as it require a bit less understanding of basic trigonometry.

preview

 using UnityEngine;
 public class FabulousAdventuresInBasicTrigonometry : MonoBehaviour
 {
     [SerializeField] float _rotationSpeed = 11;
     Vector2 mousePos = Vector2.zero;
     Vector2 mouseClickPos = Vector2.zero;
     Quaternion _rot = Quaternion.Euler( 0 , 0 , 0 );
     void Update ()
     {
         mousePos = Input.mousePosition;
         if( Input.GetMouseButtonDown(0) )
             mouseClickPos = Input.mousePosition;
         
         Vector2 dir = mousePos - mouseClickPos;
         Quaternion targetRot = Quaternion.Euler( 0 , -Mathf.Atan2(dir.y,dir.x)*Mathf.Rad2Deg , 0 );
 
         // Note: atan gives you counter-clockwise rotation from {y=0,x=1} (right dir vec)
         //       where Unity uses clockwise rotation starting at {y=1,x=0} (up dir vec)
         //       so compensate for that accordingly
 
         _rot = Quaternion.Lerp( _rot , targetRot , Time.deltaTime * _rotationSpeed );
         Debug.DrawLine( Vector3.zero , _rot*Vector3.right );
     }
 }

Mathf.Atan changes instantly even with a slow lerped value

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