changing mass of body during run time?

[yoshymoo]

Hello!
I understand that we can specify the mass of a body during initialisation. But is it possible to change the mass of a body during run time? I'm looking at the ModifyMassTest sample and I know we can do so using the contact setting. But what if there's no contact?

[jrouwe]

Use Body::GetMotionProperties to get the MotionProperties object, then use MotionProperties::SetMassProperties or MotionProperties::SetInverseMass / MotionProperties::SetInverseInertia. Note that if you change mass, then normally you should change the inertia too (in the MotionProperties header there's more information about this).

[yoshymoo]

Hello! Thank you for replying. I am currently using the SetMassProperties function, here is how I'm using it:

JPH::BodyLockWrite lock(PHYSICS->physics_system.GetBodyLockInterface(), m_BodyID);
if (lock.Succeeded()) {
	JPH::MotionProperties* motionProperties = lock.GetBody().GetMotionProperties();
	JPH::MassProperties massProperties;
	//massProperties.mMass = 10.f;
	massProperties.ScaleToMass(10.f);
	motionProperties->SetMassProperties(JPH::EAllowedDOFs::All, massProperties);
}

I'm trying not to touch the inertia so I'm letting Jolt do the math. When I print out lock.GetBody().GetMotionProperties()->GetInverseMass(), it does give me the new mass which is 10.f, but it doesn't seem like it's doing anything...? I'm trying to adjust the mass for my object movement.

Perhaps I am initialising wrongly? This is an example of how I am initialising my objects (I'm using scaled shape for all objects):

JPH::Ref<JPH::BoxShape> boxShape = new JPH::BoxShape(JPH::Vec3(1.f, 1.f, 1.f));
JPH::Ref<JPH::Shape> scaledShape = new JPH::ScaledShape(boxShape, JPH::Vec3(scale.x / 2.f, scale.y / 2.f, scale.z / 2.f));
JPH::BodyCreationSettings body(scaledShape, JPH::RVec3Arg(finalPos.x, finalPos.y, finalPos.z), placeHolder, eType, CollisionUtils::Layers::MOVING);
body.mOverrideMassProperties = JPH::EOverrideMassProperties::CalculateInertia;
body.mMassPropertiesOverride.mMass = mass;m_BodyID = PHYSICS->body_interface->CreateAndAddBody(body, activateBody);

[jrouwe]

This should work:

JPH::BodyLockWrite lock(PHYSICS->physics_system.GetBodyLockInterface(), m_BodyID);
if (lock.Succeeded()) {
	JPH::MotionProperties* motionProperties = lock.GetBody().GetMotionProperties();
	JPH::MassProperties massProperties = lock.GetBody().GetShape()->GetMassProperties();
	massProperties.ScaleToMass(10.f);
	motionProperties->SetMassProperties(JPH::EAllowedDOFs::All, massProperties);
}

But GetMassProperties can be expensive for complex shapes. I've added a new helper function ScaleToMass that makes it really simple to update the mass: #1326

[yoshymoo]

I'm a little confused as the mass doesn't seem to be doing anything for me. :(

Let's say I have Box A that I set to 100.f mass and Box B 1.f mass. When I use the add force function for both boxes, both boxes are accelerating at the same time. If I understand this correctly, the box with a larger mass will have a smaller acceleration, right? I'm also thinking that there should also be a difference in collision response if I collide both box A and box B together. Please take a look at my screen recording.

minuscule-engine-width-1920-height-1080-fps-5597-delta-time-0_dttbi7QT.mp4

I'm using this function currently

JPH::BodyLockWrite lock(PHYSICS->physics_system.GetBodyLockInterface(), m_BodyID);
if (lock.Succeeded()) {
	JPH::MotionProperties* motionProperties = lock.GetBody().GetMotionProperties();
	JPH::MassProperties massProperties = lock.GetBody().GetShape()->GetMassProperties();
	massProperties.ScaleToMass(10.f);
	motionProperties->SetMassProperties(JPH::EAllowedDOFs::All, massProperties);
}

and float currentMass = 1.0f / lock.GetBody().GetMotionProperties()->GetInverseMass(); to retrieve the body's current mass.

[jrouwe]

If I understand this correctly, the box with a larger mass will have a smaller acceleration, right?

Correct if you're using BodyInterface/Body::AddForce they will get different accelerations. If you're expecting the boxes to have different acceleration due to gravity then no, they will accelerate in the same way.

I'm also thinking that there should also be a difference in collision response if I collide both box A and box B together.

Yes, this should also be the case. The heavy box should be far less affected.

If I change the HeavyOnLightTest sample to:

void HeavyOnLightTest::Initialize()
{
	// Floor
	CreateFloor();

	Ref<BoxShape> box = new BoxShape(Vec3::sReplicate(5));

	for (int i = 1; i <= 10; ++i)
	{
		Body *b1 = mBodyInterface->CreateBody(BodyCreationSettings(box, RVec3(-75.0f + i * 15.0f, 10.0f, 0.0f), Quat::sIdentity(), EMotionType::Dynamic, Layers::MOVING));
		mBodyInterface->AddBody(b1->GetID(), EActivation::Activate);
		MassProperties m = b1->GetShape()->GetMassProperties();
		m.ScaleToMass(1000.0f);
		b1->GetMotionProperties()->SetMassProperties(EAllowedDOFs::All, m);

		Body *b2 = mBodyInterface->CreateBody(BodyCreationSettings(box, RVec3(-75.0f + i * 15.0f, 30.0f, 0.0f), Quat::sIdentity(), EMotionType::Dynamic, Layers::MOVING));
		mBodyInterface->AddBody(b2->GetID(), EActivation::Activate);
		m = b1->GetShape()->GetMassProperties();
		m.ScaleToMass(5000.0f * i);
		b2->GetMotionProperties()->SetMassProperties(EAllowedDOFs::All, m);

		b2->AddForce(1.0e7f * Vec3::sAxisZ());
	}
}

Then both scenarios you describe just work. So I'm not sure what's different in your setup.

[yoshymoo]

Well, it turns out that I needed to set the mass again after changing the scale of the bodies :)

Thank you so much!