How can i step physics and animations manually when lockstep is active?

AmirHo3ein · January 31, 2024, 7:34am

Hi, I’m developing a multiplayer game and sometimes I need to roll back in time, and process inputs for each step until I get to the current step. how can I do this? is there a method on the engine or scene object for stepping physics and animations manually when lockstep is active?

Evgeni_Popov · January 31, 2024, 10:47am

Here’s what we do to animate when deterministic lockstep is used:

github.com

BabylonJS/Babylon.js/blob/master/packages/dev/core/src/scene.ts#L4449-L4483


      
          if (this._engine.isDeterministicLockStep()) {

              let deltaTime = Math.max(Scene.MinDeltaTime, Math.min(this._engine.getDeltaTime(), Scene.MaxDeltaTime)) + this._timeAccumulator;

          

              const defaultFrameTime = this._engine.getTimeStep();

              const defaultFPS = 1000.0 / defaultFrameTime / 1000.0;

          

              let stepsTaken = 0;

          

              const maxSubSteps = this._engine.getLockstepMaxSteps();

          

              let internalSteps = Math.floor(deltaTime / defaultFrameTime);

              internalSteps = Math.min(internalSteps, maxSubSteps);

          

              while (deltaTime > 0 && stepsTaken < internalSteps) {

                  this.onBeforeStepObservable.notifyObservers(this);

          

                  // Animations

                  this._animationRatio = defaultFrameTime * defaultFPS;

                  this._animate();

                  this.onAfterAnimationsObservable.notifyObservers(this);

This file has been truncated. show original

Basically, scene._animate() will make the regular animations run (ie. all animations but physics ones) and scene._advancePhysicsEngineStep will advance the physics animations.

AmirHo3ein · February 15, 2024, 9:37am

Hello @Evgeni_Popov, I’m implementing a rollback networking model, and stepping manually this way works but there is an issue! physics animations and movements are deterministic because when I use scene._advancePhysicsEngineStep I can pass a time to it but there is no such thing for the scene._animate method and after rolling back to a specific step and stimulating to the current step the master frame of each animation group on each rollback step is different than the real step that was rendered before. is there a way to fix this? Here are some screenshots for a better understanding:

and this is my rollback logic:

Evgeni_Popov · February 15, 2024, 10:45am

This PR will allow you to pass a custom delta time to scene._animate:

AmirHo3ein · February 17, 2024, 7:39am

Thank u so much @Evgeni_Popov. I think you need to change one more thing for animations to be fully deterministic. the Default frame time must be pass to scene._animate() on line 4467.

Also, I have a question! when can I get these new changes? I checked and there is no new release that includes this PR.

Evgeni_Popov · February 19, 2024, 9:39am

I agree but that would be a breaking change… I don’t know if we can consider it a bug or not.

cc @RaananW and @sebavan for their opinion.

The merged PRs are visible on the Playground the day after they are merged. New NPM packages are created each Thursday morning (EU time).

RaananW · February 19, 2024, 10:12am

I was trying to think as well if it is a bug or not. My gut feeling is - it is a bug. Deterministic step should be deterministic. As long as we don’t change the else context I am fine with making this change, as it makes sense. If we want to be really safe here we can add a flag to the scene, but I think it will be hard to explain why we have a “isSceneDeterministicer” flag in the scene.

Evgeni_Popov · February 19, 2024, 12:20pm

I also think it’s a bug, here’s the PR:

Topic		Replies	Views
Physics Timestep Clarification Questions	5	1917	February 5, 2020
Deterministic Lockstep: Physics simulation paused while window inactive or in background Questions	2	377	December 20, 2020
Register physics step Questions	1	193	April 19, 2021
Physics as animation? Questions	1	272	June 7, 2020
New scene triggers for physics engine Feature requests	3	382	March 25, 2021

How can i step physics and animations manually when lockstep is active?

Related Topics