How to lerp a car while it's moving forward at a constant velocity

Question

This car is a rigidbody2D object that is constantly going Vector3.up * 10 while it does not collide into anything on the 2D plane. While I can move the car left and right, it does not translate smoothly. My objective is to be able to move the car left and right smoothly, but the fact that the car is constantly moving and that I can't just lerp the X-position of the car is causing me a headache.

 void Update () {
         if (Input.GetKeyDown("left"))
         {
             if (transform.position.x > -1)
             {
                 //Vector3 targetPos = transform.position;
                 //targetPos.x -= 1.25f;
                 MoveLeft ();
             }
         }
         if (Input.GetKeyDown("right"))
         {
             if (transform.position.x < 1.5)
             {
                 MoveRight();
             }
         }
     }                               
 
     void MoveLeft() {
         Vector3 targetPos = transform.position;
         targetPos.x -= 1.25f;
         transform.position = Vector3.Lerp(transform.position, targetPos, 1f);
     }

Answer 1

Best Answer

Answer by whydoidoit · Feb 15, 2014 at 07:31 AM

Use Mathf.Lerp rather than Vector3.Lerp to move just the X position of the car - then make your position a combination of the X you are lerping at the rest of it.

      float xPos = 0;
      float horizontalMovement = 2;

      void MoveLeft() {
          xPos = Mathf.Lerp(xPos, xPos-horizontalMovement, Time.deltaTime * 3);
      }

     void Update() {
         //Move vehicle upwards etc
         ...
         var pos = transform.position;
         pos.x = xPos;
         transform.position  = pos;
     }

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StrawHatGuy · Feb 15, 2014 at 06:26 PM 0

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I should have been more specific with my objective, so I apologize for the lack of detail in regards to that.

What I'm trying to do is to get the car to change lanes with just one keystroke. Eventually, I'll put the game on a mobile device so it will move into another lane with just one horizontal swipe. It would be simple to do if the car wasn't constantly moving, but the fact that it does makes it difficult to implement a solution.

whydoidoit · Feb 16, 2014 at 05:04 AM 0

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So sure - what you need for that is something like this:

  float xPos;
  float laneWidth = 4;
  float laneChangeSpeed  = 2f;

  void OnEnable() {
      xPos = transform.position.x;
  }

  void Update() {

       if(Input.Get$$anonymous$$eyDown($$anonymous$$eyCode.A)) {
           StartCoroutine($$anonymous$$oveLane(-laneWidth));
       }
       ...
       var pos = transform.position;
       pos.x = xPos;
       transform.position = pos;
  }

  IEnumerator $$anonymous$$oveLane(float offset) {
      var t = 0f;
      while(t < 1) {
           t += Time.deltaTime * laneChangeSpeed;
           xPos = $$anonymous$$athf.Lerp(xPos, xPos+offset, t);
           yield return null;
      }
  }

StrawHatGuy · Feb 16, 2014 at 05:55 AM 0

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Thank you, I will try this code as soon as I get back home.

By the way, am I supposed to adjust the offset after each lerp, or is that not necessary?

whydoidoit · Feb 16, 2014 at 05:56 AM 0

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Nope - the lerp is moving from one position to the other - it will an non-eased change however.

If you want easing we need to apply a function to the t parameter to create that effect or use an Animation curve.

How to lerp a car while it's moving forward at a constant velocity

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