How to get Body from BodyID

[mrnickzh]

Hello, im pretty new to C++ and especially JoltPhysics, so the question might be kinda stupid
I had searched the docs for BodyID to Body function, which i had found in BodyManager
I had used example Jolt wrapper that i found in the web which doesnt initialize BodyManager and i assume it is automatically initalized by PhysicsSystem
So now im kinda dumbfounded - how to get BodyManager instance or there is another way of getting Body from BodyID?

[jrouwe]

The BodyManager instance is private and cannot be accessed. The official way is described here:

https://jrouwe.github.io/JoltPhysics/#multi-threaded-access

[zkrx]

I have a related problem. I want to obtain the Body from OnContactRemoved., but only BodyIDs are given. It looks like the only way to get access to the Body is through BodyLockInterface (from your link above). However, OnContactRemoved states:

Note that this callback is called when all bodies are locked, so don't use any locking functions!

So what should I do in this situation?

[jrouwe]

As the rest of the comments state, it is not safe the access the body at this point (and it may not even exist anymore). Best way is to store the BodyID somewhere and read the body after PhysicsSystem::Update has finished.

[zkrx]

Hi jrouwe and thanks for your reply. It states that the Body might have been removed, but this is something I could check manually. Your documentation from https://jrouwe.github.io/JoltPhysics/#multi-threaded-access even shows how even though I can't use this API here (because of locking):
if (lock.Succeeded()) // body_id may no longer be valid

In any case, I need to have access to the layer associated with the Body/BodyID from within the OnContactRemoved callback. The exact same way I can access them from OnContactAdded. Is there any way to do this?

Storing BodyIDs in a vector for later access really sounds cumbersome.

		if (body1.GetObjectLayer() == Layers::SENSORS &&
		    body2.GetObjectLayer() != Layers::SENSORS)
                                     do_something();

I could work around it by storing the layer into user data, but I'd rather not.

[jrouwe]

You can use the non-locking version of the body interface:

JPH::BodyLockInterface lock_interface = physics_system.GetBodyLockInterfaceNoLock();
JPH::BodyLockRead lock(lock_interface, body_id);
if (lock.SucceededAndIsInBroadPhase())
...

but beware that the physics system may be writing to the body at the same time (mostly positions, bounding boxes and velocities). Reading the layer should be safe as the physics system doesn't change it.

[zkrx]

Ah thank you very much, I assumed based on the name that I could not use a BodyLockInterface without locking.

I've just read in [1] that the contact listeners are non-deterministic, so I will have to handle collisions outside of the physics system after all. Maybe something about this could be added in https://jrouwe.github.io/JoltPhysics/index.html#deterministic-simulation .

Cheers

[1]

JoltPhysics/Samples/Tests/General/SensorTest.cpp

Line 189 in 99ea538

// Add to list and make sure that the list remains sorted for determinism (contacts can be added from multiple threads)

[jrouwe]

Done: ddf5602