Fix constraint rotations being weird when rotate speed is fast

[IvanXuff]

QQ2025510-105716.mp4

When I use fixed constraints, I hope that there will be as little relative displacement as possible between the two physical bodies. I found that when one object rotates very fast, they will not maintain the original relative rotation, which is especially obvious when the center of mass of the two objects is far apart.

that's my test code

	auto& Floor = CreateFloor();

	float box_size = 4.0f;
	RefConst<Shape> box = new BoxShape(Vec3::sReplicate(0.5f * box_size));


	RefConst<Shape> box2 = new BoxShape(Vec3( 50,2,2 ));
	RVec3 position(0, 25.0f, 0);
	auto BDS = BodyCreationSettings(box, position, Quat::sIdentity(), EMotionType::Dynamic, Layers::MOVING);
	BDS.mAllowedDOFs = EAllowedDOFs::TranslationX | EAllowedDOFs::TranslationY | EAllowedDOFs::TranslationZ | EAllowedDOFs::RotationY;
	Body &top = *mBodyInterface->CreateBody(BDS);
	mBodyInterface->AddBody(top.GetID(), EActivation::DontActivate);
	mBodyID = top.GetID();
	top.SetAngularVelocity( Vec3(0, 20, 0));


	SixDOFConstraintSettings SixDOFConstraintSetting;
	SixDOFConstraintSetting.MakeFixedAxis(SixDOFConstraintSettings::EAxis::TranslationX);
	SixDOFConstraintSetting.MakeFixedAxis(SixDOFConstraintSettings::EAxis::TranslationY);
	SixDOFConstraintSetting.MakeFixedAxis(SixDOFConstraintSettings::EAxis::TranslationZ);
	SixDOFConstraintSetting.mPosition1 = top.GetPosition();
	SixDOFConstraintSetting.mPosition2 = top.GetPosition();
	Ref<Constraint> SixDOFc = SixDOFConstraintSetting.Create(Floor, top);
	mPhysicsSystem->AddConstraint(SixDOFc);

	Body *prev = &top;
	Quat rotation;

		position += Vec3(60, 0, 0);
		rotation = Quat::sIdentity();


	auto BDC2 = BodyCreationSettings(box2, position, rotation, EMotionType::Dynamic, Layers::MOVING);
	BDC2.mOverrideMassProperties = EOverrideMassProperties::CalculateInertia;
	BDC2.mMassPropertiesOverride.mMass = 50.0f;
	Body &segment = *mBodyInterface->CreateBody(BDC2);
	mBodyInterface->AddBody(segment.GetID(), EActivation::Activate);

	FixedConstraintSettings settings;
	settings.mAutoDetectPoint = false;
	settings.mPoint1 = position;
	settings.mPoint2 = position + Vec3(2, 0, 0);
	Ref<Constraint> c = settings.Create(*prev, segment);
	mPhysicsSystem->AddConstraint(c);

	prev = &segment;

[jrouwe]

I think this is due to the fact that everything is linearized, artist impression from the top:

Initial configuration: Yellow boxes.
After 1 time step: Green is where the angular velocity will take the COM of the 2nd box if you approximate it linearly. In actual fact it follows an arc so it should have arrived at red.

There is a process in Jolt that fixes up the errors after stepping called 'position correction'. There are two important parameters that control this: PhysicsSettings::mBaumgarte and PhysicsSettings::mNumPositionSteps. If I set the mBaumgarte close to 1 the lagging goes away. If I set mNumPositionSteps much higher it also goes away (a combination of both is also possible). Increasing the Baumgarte constant can lead to an unstable simulation, increasing the number of position steps makes the simulation more expensive.

Another trick that we use is to render objects as if they had been constrained properly. This involves shifting the visual mesh so that it matches the expected position while leaving the physics body lagging behind. When things are moving very fast, it is hard to spot the difference. When things are moving slow, physics and visuals match so there is no issue either.

[IvanXuff]

WeChat_20250515222415.mp4

Thank you for your reply. Your answer helped me figure out the reason why it happened. But I still have a video here for you to watch. Is this strange shaking also due to the above reasons?
My hand is connected to the character using sixdofconstraint. Then the hand is connected to the object using fix constraint
When I grab the two ends of this long object, it will produce the situation in the above video, but it will not happen when I grab a shorter object

[jrouwe]

From the video I can't really tell what's going on. I would suggest to turn on the constraint debug drawing to see if you see anything strange. Another thing to check is if the collision between the hand and the long object has been disabled. In the video they seem to be fighting each other (constraint pulls, collision pushes).

[IvanXuff]

Thank you! After I added collision ignore, the offset between the two objects becomes very small. Almost negligible! Thanks