Time.GameTime lagged inside FixedUpdate

[Ray_Rune]

I made a simple interpolation code using Update() and FixedUpdete(), based on deltatime after last FixedUpdate() execution.

But I met strange ratio of delta time error of GameTime between Update() and FixedUpdate().

To be simple:

public override void OnUpdate()
{
    Debug.Log($"Update, Gametime :{FlaxEngine.Time.GameTime}");
}

public override void OnFixedUpdate()
{
    Debug.Log($"Fixed Update, GameTime:{FlaxEngine.Time.GameTime}");
}

Resulted following output log:

[ 16:40:33.913 ]: [Info] Update, Gametime :506.75983
[ 16:40:33.927 ]: [Info] Fixed Update, GameTime:506.54108
[ 16:40:33.931 ]: [Info] Update, Gametime :506.77652
[ 16:40:33.944 ]: [Info] Fixed Update, GameTime:506.55774
[ 16:40:33.947 ]: [Info] Update, Gametime :506.79318
[ 16:40:33.960 ]: [Info] Fixed Update, GameTime:506.5744
[ 16:40:33.963 ]: [Info] Update, Gametime :506.80988
[ 16:40:33.977 ]: [Info] Fixed Update, GameTime:506.5911
[ 16:40:33.980 ]: [Info] Update, Gametime :506.8265
[ 16:40:33.994 ]: [Info] Fixed Update, GameTime:506.60773
[ 16:40:33.998 ]: [Info] Update, Gametime :506.8432
[ 16:40:34.011 ]: [Info] Fixed Update, GameTime:506.62442
[ 16:40:34.014 ]: [Info] Update, Gametime :506.85983
[ 16:40:34.027 ]: [Info] Fixed Update, GameTime:506.64105
[ 16:40:34.030 ]: [Info] Update, Gametime :506.87653
[ 16:40:34.044 ]: [Info] Fixed Update, GameTime:506.65775
[ 16:40:34.047 ]: [Info] Update, Gametime :506.89316
[ 16:40:34.061 ]: [Info] Fixed Update, GameTime:506.67438
[ 16:40:34.063 ]: [Info] Update, Gametime :506.90985
[ 16:40:34.077 ]: [Info] Fixed Update, GameTime:506.69107
[ 16:40:34.081 ]: [Info] Update, Gametime :506.92648
[ 16:40:34.094 ]: [Info] Fixed Update, GameTime:506.70773
[ 16:40:34.097 ]: [Info] Update, Gametime :506.94318
[ 16:40:34.111 ]: [Info] Fixed Update, GameTime:506.7244
[ 16:40:34.113 ]: [Info] Update, Gametime :506.95984
[ 16:40:34.127 ]: [Info] Fixed Update, GameTime:506.74106

Apparently, FixedUpdate() called got less value of GameTime variable than Update(), regardless of sequence of calling. Headers of logs show the stable steps of 60FPS and deltas between Update<->Update and FixedUpdate<->FixedUpdate shows correct value of delta time, but timing calculation inside FixedUpdate is lagged behind Update about ~10x frames.

Is FlaxEngine.Time.GameTime desynced between OnUpdate and OnFixedUpdate?

[charlotteemma]

I made a simple interpolation code using Update( and Fixed Updete, based on delta time after last Fixed Update spacebar clicker execution. as shown below. An example showing Fixed Update running at 50 updates per second 0.02s per fixed update) and the Player Loop running at approximately 80 frames per second.Fixed update is indeed time scaled. Simple enough to test, just set the time scale to 1/100th and print something every fixedupdate.

[Ray_Rune]

Thank you for reply.

I also suspected what you said first! Because in case of Unity, majority of physics FPS is 50 (different from variable FPS of logic and draw updates) and it is commonly desynced in world of Unity projects.

As far as I had searched, Flax engine loop checks whether the timing is for Update/Physics(Fixed)/Draw in single threaded loop, and the timings may be different. (Like most other engines)

    // Update application (will gather data and other platform related events)
    {
        PROFILE_CPU_NAMED("Platform.Tick");
        Platform::Tick();
    }
    
    // Update game logic
    if (Time::OnBeginUpdate(time))
    {
        OnUpdate();
        OnLateUpdate();
        Time::OnEndUpdate();
    }

    // Start physics simulation
    if (Time::OnBeginPhysics(time))
    {
        OnFixedUpdate();
        OnLateFixedUpdate();
        Time::OnEndPhysics();
    }

    // Draw frame
    if (Time::OnBeginDraw(time))
    {
        OnDraw();
        Time::OnEndDraw();
        FrameMark;
    }

And I also checked that both Update (OnUpdate) and Physics (OnFixedUpdate) FPS are 60, therefore timing for ‘next event broadcasting’ should be same, I thought.

Unidentical ceiling value of FPS can be common the root of the many interpolation situations, but I thought that the GameTime value retrieval is something else than that…

And everything aside, I remember that this happening had ‘fixed’ in later releases of engine.
Sorry couldn’t find the exact version changelog line…
I recreated the test set in current stable version 1.9.6605, and log is very much consistent.

[ 00:30:57.240 ]: [Info] Update, Gametime :2.3666651
[ 00:30:57.240 ]: [Info] Fixed Update, GameTime:2.3666577
[ 00:30:57.257 ]: [Info] Update, Gametime :2.3833315
[ 00:30:57.257 ]: [Info] Fixed Update, GameTime:2.3833244
[ 00:30:57.274 ]: [Info] Update, Gametime :2.399998
[ 00:30:57.274 ]: [Info] Fixed Update, GameTime:2.3999908
[ 00:30:57.290 ]: [Info] Update, Gametime :2.4166648
[ 00:30:57.290 ]: [Info] Fixed Update, GameTime:2.4166577
[ 00:30:57.307 ]: [Info] Update, Gametime :2.4333317
[ 00:30:57.307 ]: [Info] Fixed Update, GameTime:2.433324
[ 00:30:57.324 ]: [Info] Update, Gametime :2.4499981
[ 00:30:57.324 ]: [Info] Fixed Update, GameTime:2.4499907
[ 00:30:57.341 ]: [Info] Update, Gametime :2.4666648
[ 00:30:57.341 ]: [Info] Fixed Update, GameTime:2.4666572
[ 00:30:57.357 ]: [Info] Update, Gametime :2.4833314
[ 00:30:57.357 ]: [Info] Fixed Update, GameTime:2.483324
[ 00:30:57.374 ]: [Info] Update, Gametime :2.4999983
[ 00:30:57.374 ]: [Info] Fixed Update, GameTime:2.4999905
[ 00:30:57.391 ]: [Info] Update, Gametime :2.5166647
[ 00:30:57.391 ]: [Info] Fixed Update, GameTime:2.516657
[ 00:30:57.407 ]: [Info] Update, Gametime :2.5333312
[ 00:30:57.407 ]: [Info] Fixed Update, GameTime:2.5333235
[ 00:30:57.424 ]: [Info] Update, Gametime :2.549998
[ 00:30:57.424 ]: [Info] Fixed Update, GameTime:2.5499904
[ 00:30:57.440 ]: [Info] Update, Gametime :2.566665
[ 00:30:57.441 ]: [Info] Fixed Update, GameTime:2.5666568

[OlanderDA]

This has to do with how Flax handles the loop event scheduling (makes it a lot faster to do it this way too). Flax does not work like most other engines. To be honest other engines should be migrating to how Flax is doing things but will take substantial work to do it.

Fun side note, BGS Creation Engine 1/2 already does this and has for a very long time.

Per the Manual.

Script events | Flax Documentation

For the Physics loop and execution order. Hard testing. I put all the math in there too.

Physics event execution order

I am looking for a documentation for Flax’s complete scripting execution order, including Physics event (e.g : OnCollisionEnter(), etc) This is what I can find from the Manual Script events | Flax Documentation However, it doesn’t include Physics events Here is Unity’s Manual Unity - Manual: Order of execution for event functions So looking at Unity, the execution order in the frame when Physics is updated : FixedUpdate() → OnCollisionXXX(), → Update() I assume Flax is using the s…

Only in very rare cases should you (the coder) allow analysis (via code logic execution) on anything faster than 25ms. In most cases you should have a handler than does most everything on 50ms intervals (x20 per second). This is plenty fast for just about anything. Make sure to read the notes I made about Flax physics settings. Flax defaults are still not right.

This means you would have a logic handler that executes ‘scripts’ on intervals.

Sure, some things must operate on the Frame. Which frame depends on what it is and what you need to do with it (ie OnUpdate, OnLateUpdate, OnfixedUpdate…etc.). In 99% of all things done you can mostly get away with OnUpdate then the 1% that needs the interesting random behavior of Physics to release a Physics Copy of something and let it be handled by the OnFixedUpdate at Frame Loop speed. This is only very temporary and will save you a lot of headaches later.

This above philosophy works with ArchVis, FPS Shooters, Survival, Life Sims, anything really. You must build a foundational framework to make your work later easier. Flax and Unity are very easy to build this framework. UE4/5 have their own framework already and can be difficult to work within if outside of what Epic built their engine for purpose. Godot is also much harder to build a framework like this because of how the scripts are attached to their nodes.

Hope that gives some insight.