Correct Method to Shift Centre of Mass during Runtime

[08jne01]

I am running a flight model simulation for an aircraft and this requires the centre of mass and inertia tensor to be updated every frame as fuel is burned and transferred around.

If I have understood the docs correctly the offset allows you to offset the centre of mass meaning the original calculated centre of mass is used. Would the below code snippet work to set the centre of mass at a desired point in the shape frame of reference?

// aircraft_shape is a JPH::Shape::ShapeResult from the actual aircraft geometry
JPH::RefConst<JPH::Shape> offset_shape = new JPH::OffsetCenterOfMassShape( state.aircraft_shape.Get(), centre_of_mass - state.aircraft_shape.Get()->GetCenterOfMass() );

Secondly when the centre of mass is shifting during runtime I must update it and the inertia tensor:

• For the inertia tensor I presume just setting the mass properties during runtime is the intended method to do this?
• For the centre of mass update I presume setting the shape to a new OffsetCenterOfMassShape with the appropriate centre of mass, does this incure any cost? Is there a better way like modifying the existing shape and setting it?

Related discussion: #397

Any help is appreciated!

[jrouwe]

Would the below code snippet work to set the centre of mass at a desired point in the shape frame of reference?

Yes, I think that would work.

For the inertia tensor I presume just setting the mass properties during runtime is the intended method to do this?

You would do something like this to ensure that the inertia is properly set on the plane:

JPH::RefConst<JPH::Shape> offset_shape = new JPH::OffsetCenterOfMassShape( state.aircraft_shape.Get(), centre_of_mass - state.aircraft_shape.Get()->GetCenterOfMass() );
body_interface->SetShape(plane_body.GetID(), offset_shape, true /* update mass properties */, EActivation::DontActivate);

For the centre of mass update I presume setting the shape to a new OffsetCenterOfMassShape with the appropriate centre of mass, does this incure any cost? Is there a better way like modifying the existing shape and setting it?

Shapes are immutable (except for MutableCompoundShape), so there aren't many other options. Setting a new shape will incur some cost, but compared to the rest of the sim it should be minor.

[08jne01]

Since I am setting the inertia tensor and mass for empty aircraft from known values and then adding mass for fuel and such. I assume it's correct to set the mass properties and set the recalculate mass when setting the shape to false?

Here is a more complete snippet just after I have finished recalculating the inertia tensor and centre of mass (moi and com respectively).

 JPH::MassProperties mass_properties;
 JPH::Mat44& I = mass_properties.mInertia;
 I = JPH::Mat44::sIdentity();

// row 0
I(0,0) = float(qvm::A00(moi));
I(0,1) = float(qvm::A01(moi));
I(0,2) = float(qvm::A02(moi));

// row 1
I(1,0) = float(qvm::A10(moi));
I(1,1) = float(qvm::A11(moi));
I(1,2) = float(qvm::A12(moi));

// row 2
I(2,0) = float(qvm::A20(moi));
I(2,1) = float(qvm::A21(moi));
I(2,2) = float(qvm::A22(moi));
    
mass_properties.mMass = float(mass);

//JPH::MassProperties::DecomposePrincipalMomentsOfInertia(  )


JPH::BodyInterface& body_interface = state.physics_system.GetBodyInterface();
JPH::RefConst<JPH::Shape> offset_shape = new JPH::OffsetCenterOfMassShape( state.aircraft_shape.Get(), qvm::scalar_cast<float>(com) - state.aircraft_shape.Get()->GetCenterOfMass() );
body_interface.SetShape(aircraft.body_id, offset_shape, false, JPH::EActivation::Activate);

JPH::BodyLockWrite lock(state.physics_system.GetBodyLockInterface(), aircraft.body_id);

if ( ! lock.Succeeded() )
    return;

JPH::Body& body = lock.GetBody();
body.GetMotionProperties()->SetMassProperties(JPH::EAllowedDOFs::All, mass_properties);

I see the new and delete keywords are overridden for shapes is there some custom allocator that makes it cheaper to allocate these shapes under the hood? Or is this for the reference counting specifically.

Not that it matters for my use case since I don't have too many bodies , but just for my understanding what is the reason behind setting JPH::EActivation::DontActivate? Is it because this case we are only updating the COM so if the body is at rest it would remain at rest since the actual geometry has not changed. I guess though if the body is no longer balanced and would tip over with the new centre of mass would the engine automatically activate the body in this case?

Thank you for the quick response your help is very much appreciated. I can't overstate how easy this physics engine has been to use compared to others I've used in the past.

[jrouwe]

Here is a more complete snippet just after I have finished recalculating the inertia tensor and centre of mass (moi and com respectively).

That looks like it should work.

I see the new and delete keywords are overridden for shapes is there some custom allocator that makes it cheaper to allocate these shapes under the hood? Or is this for the reference counting specifically.

This is so that all new Shape calls go through the Allocate function and allows you to have your own memory allocator (if !JPH_DISABLE_CUSTOM_ALLOCATOR). By default they will go through malloc. Note that if you're really worried about the allocation, you can embed a OffsetCenterOfMassShape somewhere in your own class and placement new it. If you call SetEmbedded then you effectively disable reference counting, and prevent Jolt from calling delete on the shape.

Not that it matters for my use case since I don't have too many bodies , but just for my understanding what is the reason behind setting JPH::EActivation::DontActivate?

If you wanted to modify a sleeping body and you don't want to wake it up. For active bodies (as your plane probably is), it doesn't matter if you pass Activate or DontActivate. I picked DontActivate so that it doesn't change the current sleeping state.

I guess though if the body is no longer balanced and would tip over with the new centre of mass would the engine automatically activate the body in this case?

Nothing is automatically activated. You need to do this yourself. The reason for this is that it used to be super easy in our games to inadvertently activate tons of bodies when streaming in/out level sections causing huge frame spikes. So Jolt doesn't try to be smart and leaves the choice to you.