Let's say that you have a Rigidbody
Object that moves. Force is added to this Object via Rigidbody.AddForce
or Rigidbody.velocity
. The Object can roll hit another Object and change direction.
I know about Extrapolation but in this case, it's nearly impossible to use some formula to obtain the position of the object in x seconds, since the Object can hit another object and change speed/direction in the process.
Unity 2017 introduced Physics.autoSimulation
and Physics.Simulate
to solve this problem. For 2D physics, that is Physics2D.autoSimulation
and Physics2D.Simulate
. All I did was first set Physics.autoSimulation
to false then call the Physics.Simulate
function.
In my example, I wanted to know where a Rigidbody
would be in 4
seconds after adding force to it, it seems to work fine for tiny seconds like 1
. The problem is that when I pass in bigger numbers like 5
and above, to the Simulate
function, the predicted position is not accurate. It's way way off.
Why is this happening and how can I fix it? This problem is worse on Android devices.
My current Unity version is Unity 2017.2.0b5.
Below is the sample code I am using. The guide
GameObject is simply used to display/show where that predicted position is.
public GameObject bulletPrefab;
public float forceSpeed = 50;
public GameObject guide;
// Use this for initialization
IEnumerator Start()
{
//Disable Physics AutoSimulation
Physics.autoSimulation = false;
//Wait for game to start in the editor before moving on(NOT NECESSARY)
yield return new WaitForSeconds(1);
//Instantiate Bullet
GameObject obj = Instantiate(bulletPrefab);
Rigidbody bulletRigidbody = obj.GetComponent<Rigidbody>();
//Calcuate force speed. (Shoot towards the x + axis)
Vector3 tempForce = bulletRigidbody.transform.right;
tempForce.y += 0.4f;
Vector3 force = tempForce * forceSpeed;
//Addforce to the Bullet
bulletRigidbody.AddForce(force, ForceMode.Impulse);
//yield break;
//Predict where the Rigidbody will be in 4 seconds
Vector3 futurePos = predictRigidBodyPosInTime(bulletRigidbody, 4f);//1.3f
//Show us where that would be
guide.transform.position = futurePos;
}
Vector3 predictRigidBodyPosInTime(Rigidbody sourceRigidbody, float timeInSec)
{
//Get current Position
Vector3 defaultPos = sourceRigidbody.position;
Debug.Log("Predicting Future Pos from::: x " + defaultPos.x + " y:"
+ defaultPos.y + " z:" + defaultPos.z);
//Simulate where it will be in x seconds
Physics.Simulate(timeInSec);
//Get future position
Vector3 futurePos = sourceRigidbody.position;
Debug.Log("DONE Predicting Future Pos::: x " + futurePos.x + " y:"
+ futurePos.y + " z:" + futurePos.z);
//Re-enable Physics AutoSimulation and Reset position
Physics.autoSimulation = true;
sourceRigidbody.velocity = Vector3.zero;
sourceRigidbody.useGravity = false;
sourceRigidbody.position = defaultPos;
return futurePos;
}
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