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Physics Sub-Stepping

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    #16
    To add to the example toxygen posted, it is OK to get the world transform from the BodyInstance directly.
    You can also apply forces, torque, and more on the BodyInstance, but you will want to pass the optional flag that says that you are already sub-stepping in cases that need it (like AddForce)

    void AddForce(const FVector& Force, bool bAllowSubstepping = true);

    In the case of calling from sub-stepping code you'd want to explicitly pass bAllowSubstepping as false. An example of this can be found in UWheeledVehicleMovementComponent::UpdateDrag

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      #17
      Physics sub-stepping is a great feature. Having the "free the physics" approach would be much better than the semi-fixed timestep however in my opinion. All physics-based game/simulation would greatly benefit of the fixed timestep and the reproducible physics of the "free the physics" approach. But it's still an interesting feature.

      One question though: do you plan to expose the "physics tick" of the sub-stepping in Blueprint ? That would be extremely useful. I have a case where enabling sub-stepping without having access to the physics tick breaks things... I'm going to explain it so you can have an actual example and might see the value in exposing it.

      I'm trying to prototype a UAV simulator (quadcopter for now) using Blueprints. The modelling of the quadcopter is physics-based. What I mean by that is that the quadcopter has 4 thrusters (imagine a X shape with a propeller at each corner and I placed a thruster at each propeller's location). The motion of the quadcopter is entirely controlled by the strength set for the thrusters. In this situation, if the object on which I have my 4 thrusters attached (my mesh) is perfectly balanced (if my center of mass is exactly at the center of the 4 thrusters), setting the same thrust value for each propeller will keep my quadcopter perfectly horizontal (no roll and no pitch). On the other hand, if I want to turn right, I increase the thrust on the left propellers and reduce the thrust on the right propellers. However, as with actual quadcopters, if I send a command to the quad to turn left by 10 degrees, I have no way to know how much thrust is needed to achieve 10 degrees (it's a non-linear problem and it might depends on external conditions like wind for instance). So the solution is to implement PID controllers on each axis to control the motion correctly. For those not familiar with PID controllers, this is the idea behind it: let's say I ask for a 10 degrees angle to the right (so target = 10 degrees) and my quadcopter is currently horizontal (so actual = 0 degree). We calculate the error (target - actual) and we use this error and some PID coefficients to compute a correction (the simplest case would be to compute a correction = constant * error). In my case, I produce a correction to modulate the propeller speeds, which in turn changes the thrust produced by each propeller. The next frame, the quadcopter will have started to move right, and it might be at 6 degrees. So the new error would be 10 - 6 = 4, and the new correction for this frame would be smaller than the previous frame so we can converge towards our target with a smaller and smaller correction until no correction is no longer needed when we are on target.

      My control works great and the quadcopter moves as expected. I compute the error between the command (target) and the actual value (current orientation of the quadcopter), use my PID controller to generate a correction to be applied to the propeller speed, which varies the thrust of each propeller and correct the motion of my quadcopter to converge towards the target. To check that, I asked a target of 1 degree and looked at all the values in my simulation at each frame. In less than 10 frames the actual angle was on target (with a decreasing speed when approaching the targeted angle).

      So here's my problem now. When I came across this physics sub-stepping features (and the promise to get an improved physics), I turned it on. And my control stopped working. First, the actual angle seems to converge towards my target of 1 degree, but actually went much beyond the target. When I observe the speed at which it converged towards the target, it first slowed down (as expected since the error decreases, so the amount of correction at each new frame should decrease too) but after a few frames (5 or 6), the speed at which it converged increased. Basically, it does not work.

      My guess is that the force is applied at each sub step, the new position/orientation is computed, and my physics simulation is going forward much faster, right? And since I'm doing my control only once by frame, the motion of my quadcopter basically goes uncontrolled during the sub-steps process. I compute a correction based on the current error right before starting a frame. With physics sub-stepping on, after one sub-step, my quadcopter had updated its position/orientation. Even though it's now closer to the target, the same correction is applied once again (based on the position at the beginning of the frame). Instead, the correction should be reduced after each sub-step since we are getting closer to the target. If I had access to the physics sub-steps, I could update the correction between each sub-step.

      Being able to implement the control within each sub-step would also allow it to be executed faster than only at each frame. In real life, the control loop of a quadcopter is executed at frequencies ranging from 100 Hz to 500 Hz. The quadcopter seems more responsive and the quality of the control is better. Doing the control in Unreal Engine at a higher frequency will see my quad to be more responsive too.

      I think that giving users more control on the physics (and adding features like sub-stepping) is crucial for serious games. I'm new to UE4 and so far I'm very impressed by it. Keep the great work! We have a custom engine for doing surgical real-time simulation and we consider using UE4 for a few new projets we have (like this UAV simulator). Being able to implement our own physics in a decoupled way from the rendering is crucial. So I just wanted to let you guys know about that.

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        #18
        You can try to avoid the tick for your PID controller and use a timer instead. The timer can tick at 100 Hz, probably much faster. This will maybe help a lot, but you still can't predict physics exactly - Sadly, the time when the physics calculation starts is unpredictable.

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          #19
          Is it possible to access these sub-steps in blueprints? Some work is better done during the physics sub-step.

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            #20
            Originally posted by Wolfsblut View Post
            You can try to avoid the tick for your PID controller and use a timer instead. The timer can tick at 100 Hz, probably much faster. This will maybe help a lot, but you still can't predict physics exactly - Sadly, the time when the physics calculation starts is unpredictable.
            Thanks. It seems like a good idea intially. But it wouldn't work un my case. Executing my control faster is not sufficient. I need to be able to retrieve the position of my object at a faster rate too and it's my understanding that the position is not updated between sub-steps. So I would need access to the tick of the sub-steps, the PhysX position and the ability to add force to the PhysX object too. I guess if I need to increase the frequency of my control (I have good results once per frame for now, even though I don't like the fact it's framerate dependent) I will go the C++ route then...

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              #21
              Originally posted by toxygen View Post
              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?

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                #22
                Hi Ori, 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).

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                  #23
                  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.

                  Originally posted by toxygen View Post
                  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:
                  Code:
                  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:
                  Code:
                  FCalculateCustomPhysics OnCalculateCustomPhysics;
                  OnCalculateCustomPhysics.BindUObject(this, &UTrainPhysicsSimulator::CustomPhysics);
                  The delegate body is:
                  Code:
                  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 irc.freenode.net #unrealengine (nickname BlackFox), this isn't the correct account from which I should be replying.

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                    #24
                    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.

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                      #25
                      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.

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                        #26
                        I made a tutorial here about making a fixed timestep, that might be the thing you people are missing

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