Physics Sub-Stepping

[anonymous_user_14f95067]

@;224098

Yes, for our project we have added that feature and it should be present in engine versions 4.6 and up.

Could you post a more extensive example?

[old_xia]

Hi I have a question: Do you think it is possible, to substepping physic with a constant delta time, and at the end interpolate the left over time? (just used in this frame, discard it and recalculate in next update).

[Vaei]

How do you enable substepping and tie it to an update now? This no longer works, the BodyInstance->GetPxRigidBody(); is always nullptr and the bound function CustomPhysics() is never called. Yeah, substepping is enabled in project settings -> physics.

Epic please document this properly.

@;224098

Yes, for our project we have added that feature and it should be present in engine versions 4.6 and up.

In the tick of component which you want to access sub-steps from the game, do this:

void URailroadWheelComponent::TickComponent(float DeltaTime, enum ELevelTick TickType, FActorComponentTickFunction *ThisTickFunction)
{
	Super::TickComponent(DeltaTime, TickType, ThisTickFunction);

	// Add custom physics forces each tick
	GetBodyInstance()->AddCustomPhysics(OnCalculateCustomPhysics);
}

This will call the delegate ‘OnCalculateCustomPhysics’ during next tick, for each substep (16 times for 16 substeps). Define delegate as:

FCalculateCustomPhysics OnCalculateCustomPhysics;
OnCalculateCustomPhysics.BindUObject(this, &UTrainPhysicsSimulator::CustomPhysics);

The delegate body is:

void UTrainPhysicsSimulator::CustomPhysics(float DeltaTime, FBodyInstance* BodyInstance)
{
	URailroadWheelComponent* RailroadWheelComponent = Cast<URailroadWheelComponent>(BodyInstance->OwnerComponent.Get());
	if (!RailroadWheelComponent) return;

	// Calculate custom forces
	//.....

	// You will have to apply forces directly to PhysX body if you want to apply forces! If you want to read body coordinates, read them from PhysX bodies! This is important for sub-steps as transformations aren't updated until end of physics tick
	PxRigidBody* PRigidBody = BodyInstance->GetPxRigidBody();
	PxTransform PTransform = PRigidBody->getGlobalPose();
	PxVec3 PVelocity = PRigidBody->getLinearVelocity();
	PxVec3 PAngVelocity = PRigidBody->getAngularVelocity();

	//......

	PRigidBody->addForce(PForce, PxForceMode::eFORCE, true);
	PRigidBody->addTorque(PTorque, PxForceMode::eFORCE, true);
}

You can contact me at #unrealengine (nickname ), this isn’t the correct account from which I should be replying.

[Yata]

Enable Physic Simulation for body component. CustomPhysics should be called. For Actor, better use this function for Actors::Tick, we had some artifacts using it in Component.

[JacobPL]

Can I enable and disable sub-stepping dynamicly? I have throwing logic where enemy can throw FPP Character on ragdoll and everything looks fine if there is no frame drop. I want to use sub-stepping only when this situation occure, not on default because i want to save CPU usage on other stuff.

[anonymous_user_70694eae1 (anonymous_user_70694eae)]

I made a tutorial here about making a fixed timestep, that might be the thing you people are missing

[anonymous_user_71818e5d]

Hey is there any chance you could send me those files? Thank you!

[TheFiftGuy]

@anonymous_user_d573ef49 sry to revive an old thread but does this still work in UE 5.1? With the switch to the Chaos physics engine, it seems that @anonymous_user_eebd5ec2 's example of interacting with the SubStep Tick is no longer possible. Is there an alternative?

Or does Chaos somehow make it that this functionality (custom Tick) is no longer needed?

[anonymous_user_71818e5d]

You can use Async Physics Tick, in which there is an additional node in actor bps for using the fixed timestep

[tobiasthorsen]

Using UE 5.1 - I managed to hook on to the c++ physics substeps using various guides online, but nothing gave me substeps. It calls my physics substep in sync with tick.
Is this functionality defuct? For my spaceship, i either have to live with exploding physics or slow motion on low framerate. Neither seems reasonalble.

My setup:

UPhysicsSettings::Get()->bSubstepping = true;
    OnCalculateCustomPhysics.BindUObject(this, &USpaceship::PhysStep);
}

Tick:

UPrimitiveComponent* PrimitiveComponent = Cast<UPrimitiveComponent>(Owner->GetRootComponent());
    PrimitiveComponent->GetBodyInstance()->AddCustomPhysics(OnCalculateCustomPhysics);
 UE_LOG(LogTemp, Warning, TEXT("TickComponent %f"), DeltaTime);

PhysStep:

void USpaceship::PhysStep(float DeltaTime, FBodyInstance* BodyInstance)
{
    UE_LOG(LogTemp, Warning, TEXT("PhysStep %f"), DeltaTime);
}

The output when console t.maxFPS 60

LogTemp: Warning: PhysStep 0.023294
LogTemp: Warning: TickComponent 0.023294
LogTemp: Warning: PhysStep 0.026082
LogTemp: Warning: TickComponent 0.026082

Output when t.maxFPS 30

LogTemp: Warning: TickComponent 0.033334
LogTemp: Warning: PhysStep 0.033334
LogTemp: Warning: TickComponent 0.033334
LogTemp: Warning: PhysStep 0.033358

My maximum step size is set to 1/60 and i would expect several PhysTicks pr frame. Am i misunderstanding something?

I have of course been in the settings and tried all combinations of stepsizes anbd options I could think of.

[tobiasthorsen]

Just a note, i got it to work using a plugin this generous guy provided here:

Exposing UE5 TickPhysicsAsync Event

I’ve been doing some more digging in the source code and the chaos vehicle implements asynchronous physics by using its own physics scene and, by the looks of it, manages that scene within the vehicle manager. Is that what it takes?! … I might be in over my head.

After a little tweaking I got fixed size async physics stepping and my spaceship does not explode any more.

[bababhuvaneshus]

The physics simulation is then ticked multiple times per frame. The number of sub-steps taken will depend on how small your max sub-step delta time is set to.