Game behaves differently on different frame rates despite using Time.deltaTime

Unity Engine
,
1

Hi everyone!

In my (android) game I use Vector3.Lerp inside a coroutine loop to move an object from one position to another. However, even though I use Time.deltaTime, the object moves faster when I run the game at 120fps instead of 60fps.

Here is the main code that handles the movement of objects:

public class ObjectController : MonoBehaviour
{
    public GameObject pointA;
    public GameObject pointB;
    Vector3 startingPosition;
    Vector3 targetPosition;
    //Time it takes to reach the end position
    public float timeToLerp;

    public UIManager uiManager;

    private void Start()
    {
        AssignPositions();
    }

    private void AssignPositions()
    {
        startingPosition = pointA.transform.position;
        targetPosition = pointB.transform.position;
    }

    public void MoveObject()
    {
        transform.position = startingPosition;
        StartCoroutine(MoveObjectRoutine());
    }

    IEnumerator MoveObjectRoutine()
    {
        bool hasReachedTargetPosition = false;
        float elapsedTime = 0f;

        while (!hasReachedTargetPosition)
        {
            if (Vector3.Distance(transform.position, targetPosition) < 0.001)
            {
                transform.position = targetPosition;
                hasReachedTargetPosition = true;
                if (Application.targetFrameRate == 60) uiManager.timeTook60Text.text = elapsedTime.ToString();
                else if (Application.targetFrameRate == 120) uiManager.timeTook120Text.text = elapsedTime.ToString();
                else if (Application.targetFrameRate == 30) uiManager.timeTook30Text.text = elapsedTime.ToString();

                Debug.Log(elapsedTime);

                break;
            }

            elapsedTime += 2 * Time.deltaTime;
            float t = elapsedTime / timeToLerp;

            transform.position = Vector3.Lerp(transform.position, targetPosition, t);

            yield return null;
        }
    }
}

Here are the results when I run the game on my mobile device, where Time Took for each frame rate displays the elapsedTime when the object has finished Lerping:

7994289--1027437--Screenshot_20220325-183428_FPS Test.png
7994289--1027437--Screenshot_20220325-183428_FPS Test.png1080×2400 113 KB

As you can see the object reaches its target position faster the higher the frame rate is. Why is that? Am I doing something wrong?

Essentially, what I am trying to achieve is for the object to take the same amount of time to Lerp between two positions, no matter what the frame rate is.

Am I missing something? Isn’t Time.deltaTime supposed to make your code frame rate independent? Why does it behave differently on different frame rates?

Thank you for your time!

2

You’re doing your Lerp incorrectly, which is introducing some framerate dependency into he mix. you have this:
transform.position = Vector3.Lerp(transform.position, targetPosition, t);But the correct way to do it is this:transform.position = Vector3.Lerp(startingPosition, targetPosition, t);Where startingPosition is the position your object started at, which you should save at the beginning of the coroutine, before the loop starts. It should not change over time.

1 Like
3

This is it! From the bottom of my heart, thank you!