koobas.hobune.stream
Centre of Mass not recalculated after scaling
I am spawning a prefab object at runtime (actually, in the Start() method of another object), and I need to apply a scaling to the object. I made a little component to handle this:
public class Spawner : MonoBehaviour {
public Transform SpawnPrefab;
public Vector3 Scale;
void Start () {
var spawn = Instantiate(SpawnPrefab, Vector3.zero, Quaternion.identity);
spawn.localScale = Vector3.Scale(spawn.localScale, Scale);
}
}
The pivot point of the prefab I am spawning does not coincide with the centre of mass of the object. Therefore, the scaling means that the centre of mass location relative to the pivot will change. However, it's not being updated automatically, so my spawned object has unexpected physics.
I tried adding a call to GetComponent<Rigidbody>().ResetCenterOfMass(); immediately after the call to Scale(), but this has no effect.
However, if I put the call to ResetCenterOfMass(); in the Update() method of a separate little component added to the spawned object, this does cause the centre of mass to be recalculated correctly.
Why isn't the COM being automatically recalculated, without me having to call ResetCenterOfMass() explicitly? And if I must trigger it manually, is there any way I can do that immediately after the calls to Instantiate() and Scale(), rather than waiting until Update()?
With thanks to DMGregory on GameDev for the suggestion, a call to `Physics.SyncTransforms` before invoking Rigidbody.ResetCenterOfMass fixes the problem:
public class Spawner : MonoBehaviour {
public Transform SpawnPrefab;
public Vector3 Scale;
void Start () {
var spawn = Instantiate(SpawnPrefab, Vector3.zero, Quaternion.identity);
spawn.localScale = Vector3.Scale(spawn.localScale, Scale);
Physics.SyncTransforms();
spawn.GetComponent<Rigidbody>().ResetCenterOfMass();
}
}
Evidently this direct modification of the transform scale isn't being automatically passed through to the physics engine, but Physics.SyncTransforms lets us manually flush those changes down to PhysX, so that the ResetCenterOfMass computation is then based on the correctly scaled transform.
