Body Instance

About

Basics

See Primitive Component for extra info on how they work together.

A important parent class of the BI is FBodyInstanceCore The body instance seems to be mostly used from UPrimitiveComponent. It also seems that a lot of physic functions are running from FBodyInstance. BIs are initialized using a UBodySetup.

Physics Core

See Physics types for more detailed info on some types

Most of the functions will register a physic command using ApplyAsyncPhysicsCommand, that has a function call-back that’s using FPhysicsInterface (a typedef of FPhysInterface_Chaos which inherits FChaosEngineInterface and FGenericPhysicsInterface).

A BI holds a FPhysicsActorHandle ActorHandle which is the “Internal physics representation of our body instance”.

In some functions such as AddImpulse the params will be redirected to FChaosEngineInterface where either FRigidBodyHandle_Internal or FRigidBodyHandle_External handle is used (this is taken from the FPhysicsActorHandle BI handle). By “internal” we mean that we are running the code on the physics thread, external would be if we are on the game thread. Both of these handle are based on TThreadedSingleParticlePhysicsProxyBase which holds the interface to update physics properties on the particle such as (in this example) the linear velocity.

Miscs

Sleeping and wake

WakeInstance and PutInstanceToSleep will at some point call SetObjectState in TPBDRigidParticle.

Inside SetObjectState it is written than when we put an object to sleep, its velocity is zeroed and buffered (“in case the velocity is queried during sleep, or in case the object is woken up again”). After that its said that “If another force is added after the object is put to sleep, the old forces
will remain and the new ones will accumulate and re-dirty the dynamic properties which will
wake the body.”

Gravity

Warning

If you enable gravity the BI will wake up

Changing collision

For some reasonSetCollisionEnabled(NewState) doesn’t work as expected, you can use SetShapeCollisionEnabled(0, NewState) instead. For more detailed collision setting, there are many functions like SetResponseToChannel and SetResponseToAllChannels.

Warning

SetShapeCollisionEnabled calls UpdatePhysicsFilterData, which will wake up the BI

See also Collisions

Constraint

When you select a PrimitiveComponent, you can set Constraints

These constraint settings are ONLY applied on the first body instance of this component For a regular static mesh, this would be expected, but for a skeletal mesh component, this means only the root bone is constrained.

To apply constraint to other body instances, one way is to iterate the Bodies (available in SKMC) and apply set the constraint you want Example:

// iterate all the bodies of this Static Skeletal Mesh Component
for (int i = 0; i < Bodies.Num(); ++i)  
{  
    FBodyInstance* BI = Bodies[i];  
 
	// Those settings are almost the same that you can see in the details panel
    BI->DOFMode = EDOFMode::Type::Default;
    // Here we are locking all axis for translation
    BI->bLockXTranslation = true;
    BI->bLockYTranslation = true;
    BI->bLockZTranslation = true;
    // Must be called so the BI create a constraint in the physic engine (it will read the variables we just set)
    BI->CreateDOFLock();
}

Set body transform

If called on a dynamic (not kinematic) body, it will eventually call SetGlobalPose_AssumesLocked. And it seems that the new scale is not used there.

As mentioned in this blog post section (see FPushPhysicsData) FBodyInstance::SetBodyTransform will eventually call FSingleParticlePhysicsProxy::SetXBase.

Sleeping

SetXBase will wake up the particle if its sleeping.