Hello! I’m new to Unity and I think I’m not getting how Physics2D.simulate works.
I’m trying to implement a trajectory prediction in a 2D outer space game with line renderer using the physics simulation method presented here: Unity Trajectory Prediction (Simulation Method) | by Austin Mackrell | Medium
I implemented as in the post: a physicscene copy of the actual scene is created, clones of the planets and the player are moved there.
Then I run physicsScene2D.simulate method X iterations in the future to make my prediction when the player aims and the line renderer draws at each iteration the simulated player’s position.
Here is the simulated launch code:
public void SimulateLaunch(Transform player, Vector3 force)
{
_simulatedPlayer.transform.position = player.position;
_simulatedPlayer.transform.rotation = player.rotation;
_simulatedPlayer.GetComponent<Rigidbody2D>().velocity = Vector3.zero;
if(_lastforce != force)
{
_simulatedPlayer.GetComponent<Rigidbody2D>().AddForce(force);
for (int i = 0; i < _maxSimulSteps; i++)
{
_physicsScene.Simulate(Time.fixedDeltaTime);
points[i] = _simulatedPlayer.transform.position;
trajectoryLine.SetPosition(i, points[i]);
}
}
_lastforce = force;
}
However, I have GameObjects representing planets that exert a gravity pull on my player.
void FixedUpdate()
{
float distance = Vector2.Distance(player.transform.position, transform.position);
if (distance < influenceRange)
{
pullForce = (transform.position - player.transform.position).normalized * intensity * fixedUpdateCompensator / distance;
player.GetComponent<Rigidbody2D>().AddForce(pullForce, ForceMode2D.Force);
}
float distanceToSim = Vector2.Distance(playerSimulation.transform.position, transform.position);
if (distanceToSim < influenceRange)
{
playerSimulation.GetComponent<Rigidbody2D>().AddForce(((transform.position - playerSimulation.transform.position).normalized * intensity * fixedUpdateCompensator / distanceToSim), ForceMode2D.Force);
}
}
Every FixedUpdate, these planets calculate the force to exert using a simplified newton’s law and addForce to the player’s rigidbody.
When aiming: the line renderer is not curved when near to a planet.
But in my actual scene the ship is gravity pulled correctly.
I think i’m maybe missing something about the physics simulate lifecycle and callbacks handling ?
The fixedUpdate seems to be called because if I log inside:
if (distanceToSim < influenceRange)
{
//Log something
}
It pops in the console.
Any help appreciated, even if it’s " read this part of the documentation" 
