How to check distance between two different vectors?

Question

So I have a very basic racing game, where I would like the car to stop once it hits the finish line. Respectively when the distance between the car and the finish line hits or goes under 0 the car stops, but instead of stopping it continues to go on. How can I fix this?

Here's my code:

    public float speed = 0f;
    public float gravity = 0f;
    public Transform finishline;
    public Transform car;
    public Transform finish;


    void CarMove () 
    {
        if (Vector2.Distance (finishline.position, car.position) != 0) 
        {
            if ( Input.GetKey(KeyCode.RightArrow))
            {
                transform.Translate(Vector2.right * speed * Time.deltaTime);
            }

            else
            {
                return;
            }
        }
    }

    // Update is called once per frame
    void Update () 
    {
        CarMove();
    }
}

Answer 1

The problem your having is that motion in games, as smooth as it might look, is better seen as a series of short teleportations. So, since distance is an unsigned value (always greater than zero), your car is just teleporting right past the absolute distance of zero. That causes the check to never go off.

You have 3 options:

1. You can add a margin of error (eg. if (distance < 0.2f) instead of d != 0) But this can be a problem because if the car is moving too fast, or is too far to the left of your finish lines "point", then it still teleports past.

2. You can scrap the idea of measuring distance all together, taking advantage of Unity's OnTriggerEnter functionality to filter out the finish line/checkpoints and determine the cars victory or place. That would allow you to add in checkpoints or finish line's simply by dragging a prefab from the editor (it's always nice to have that mobility)

   Fun Note: This rout should also be more stable because of sweep collision testing. Most game engines, before they teleport an object from point A to point B, do a sort of collision test that checks for collisions with objects between the points A and B. While your standard functions are only called at point A or B for performance reasons, the OnTriggerEnter will be called even if the object is just teleported to the other side.

3. Rather than keeping track of the distance, try keeping track of how it's changing. If you're getting closer and closer, and then suddenly getting farther and farther, then you probably crossed the finish line. This can lead to a false positive though. A 360 degree turn for example, would trigger this.

Answer 2

This wouldn't answer the question of measuring the distance but for the scenario that you described, I find that placing a collider as trigger on the finish line and checking OnTriggerEnter in your car would be a better solution rather than calculating the distance.

For distance check, Dexyne is totally correct but you could also check using the sqrMagnitude which yield better performance.

You would do a check like that (finishline.position - car.position).sqrMagnitude < Mathf.Epsilon

Answer 3

Augment your finish line point with a direction vector representing the "forward" direction. Then, instead of:

if (Vector2.Distance (finishline.position, car.position) != 0) 
[...]

Do:

if (Vector2.Dot(car.position - finishline.position, finishline.direction) > 0) 
[...]

Note, if the direction happens to be aligned to a cartesian axis, say +X, the above is equivalent to:

if (car.position.X - finishline.position.X > 0) 
[...]

Answer 4

Update – My bad, as @DMGregory says the value of a distance cannot be negative (and it makes sense). As @Kimserey and I says, you can use a trigger. Probably better in this case and easier to use.

---

when the distance between the car and the finish line hits or goes under 0

I think the problem is your if statement.

Your test should be:

if (Vector2.Distance (finishline.position, car.position) <= 0)

and not if (Vector2.Distance (finishline.position, car.position) != 0) as a vector can be negative.

Note: You can also use a trigger collider on the finishing line and set the car speed to 0 when it hit the finishing line.