What is the proper way of instantiating an entity prefab with child physics bodies?

[daniel-holz]

@alemnunez

@daniel-holz ok, that’s what I am doing. But for some reason, running the exact same prefab instantiation code in BeginInitializationSystemGroup causes a mess, and doing it in between FixedStepSimulationSystemGroup and the TransformSystemGroup works just fine. I thought it was because the LocalToWorld wasn’t updated yet, but it seems like when they don’t have a parent they should just be using the LocalTransform component directly for physics.

I will try to double check again moving it to BeginInitializationSystemGroup and see what’s going on there and what components are assigned to each entity.
The joints are added as a child of the rigid body, so maybe that joint entity has a parent set and is causing LocalToWorld issues?

Honestly I am not sure what happens with joints as child entities. When baking this doesn’t happen unless I am mistaken.
Maybe because they are children of a dynamic entity, they automatically also obtain transformation components, but that would surprise me when runtime created.
The transforms on joint entities are not considered. So moving them or not makes no difference.

As to where to do your transformation or instantiation, I suggest the AfterPhysicsSystemGroup. That’s a safe moment for pretty much anything physics related. Alternatively, the BeforePhysicsSystemGroup would work as well I think.

[w34edrtfg]

So we should do something like this?

LocalTransform parentTransform = LocalTransform.FromPosition(x * blockSize, 0, y * blockSize);
Entity blockEntity = state.EntityManager.Instantiate(prefab);

state.EntityManager.SetComponentData(blockEntity, parentTransform);

var elements = state.EntityManager.GetBuffer<LinkedEntityGroup>(blockEntity);

foreach(var element in elements) {
    if (element.Value != blockEntity && !state.EntityManager.HasComponent<Parent>(element.Value)) {
        var childTransform = state.EntityManager.GetComponentData<LocalTransform>(element.Value);

        state.EntityManager.SetComponentData(element.Value, childTransform.TransformTransform(parentTransform));
    }
}

Edit: fixed by Alejandro, i was missing the part where i have to check if the entity is linked to a parent.
ty!!

[daniel-holz]

Great! I’m glad it worked.
And many thanks for sharing the working code for others which have the same question to benefit from.