Res<Time> is unreliable / jittery

[inodentry]

Bevy version: 0.7 and current main.

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Bevy's Time resource, which is the standard and recommended source of timing information in Bevy, that everything should use for anything timing-related, is inaccurate in typical game situations.

The issue occurs, because Time is updated in a system at the beginning of the First stage in the main app schedule, using std time.

This might be fine for a pure cpu simulation use case, where the app just runs as fast as possible on the CPU. However, it is inadequate for games.

The exact instant when Time is updated depends on when the time update system happens to be scheduled to run. It is not tied to actual rendering frame timings in any way.

For animation use cases (this includes most practical uses of delta time! moving objects, moving the camera …), really anything where the timing information is needed to control what is to be displayed on the screen, the Time resource will not accurately represent the actual frame timings. This could lead to artifacts, like jitter or hiccups.

Usage of "fixed timestep" is also affected, as it derives its fixed updates from Time.

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The issue can be observed if we enable vsync (which should lock Bevy to the display refresh rate and result in identical timings every frame), and print the value of time.delta() every frame update. The result is something like this:

...
16.688ms
16.560ms
16.807ms
16.487ms
16.811ms
16.635ms
17.327ms
15.999ms
16.980ms
16.151ms
16.895ms
16.637ms
16.681ms
16.628ms
16.629ms
16.734ms
16.739ms
16.501ms
16.731ms
16.765ms
16.711ms
16.528ms
16.544ms
16.921ms
16.561ms
16.530ms
16.732ms
16.699ms
...

You can see that these timings vary by at least a few hundred microseconds every frame, sometimes even as much as ~1ms from frame to frame. This is, i guess, small enough that nobody has raised an issue in Bevy yet :D … but definitely large enough to risk causing real issues with animation/physics/etc.

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The real solution to this problem would be to use frame presentation timings (time reported by the OS/driver corresponding to when the rendered frame is actually sent to the screen), which requires support from the underlying graphics APIs. wgpu does not yet provide anything for this use case. This is understandable, as there is no standard API in Vulkan yet either. AFAIK, only Android, and maybe recent versions of DirectX, and i think some Mesa extensions on linux, provide such functionality.

Relevant work in Vulkan: KhronosGroup/Vulkan-Docs#1364

In the meantime, we should explore ways to improve the accuracy of Time in any way we can. The First stage does not seem like the best place to do it.

Maybe a value that is much closer to the true frame timings (and likely good enough for most use cases) could be obtained from the Render schedule somehow? Maybe in the Prepare stage when the swapchain texture is obtained (as this is where the "vsync wait" happens)?

Please discuss.

Related issue: #3768

[superdump]

There is a great Unity article about this in conjunction with pipelined rendering and their very long hunt to straighten it out.

https://blog.unity.com/technology/fixing-time-deltatime-in-unity-2020-2-for-smoother-gameplay-what-did-it-take

[nfagerlund]

Awesome, glad the community is taking this seriously. Trying to chase down these kinds of stutter issues at the application level can leave one feeling extremely haunted.

I thought I ran into this type of jitter at a visible level in a toy Bevy project where the debug build was running at opt-level 1. Measuring exactly what's going on with this sort of thing is currently a bit beyond my abilities, so I can't say for certain what I was seeing, but as an experiment I tried adding a system to PreUpdate that did a rolling average of delta times (inspired by this ancient post from the Our Machinery folks), and then using the resulting smoothed delta for all my movement simulation. That actually seemed to help a great deal, which is at least some evidence that inaccurate delta was the source of the stutter I was seeing.

Later, I ripped that out and just flipped to opt-level 2 for debug profile, which also smoothed out the worst of the stutter. So maybe the inaccuracy gets amplified when runtime performance gets worse, or maybe I was off-base entirely; definitely not a zone where I'm confident in my judgements yet.

[aevyrie]

I ran into this when implementing bevy_framepace. The solution I found was to track time in RenderStage::Cleanup. While not perfect, I found that this ended up giving me the best perceived results as it is almost entirely dependent on when the frame is sent to the GPU, and there is very little scheduling variability to contend with. I agree that having presentation timings would be the ideal solution.

[hymm]

This seems to be related to #4691. Since we're setting time in First the timing gets delayed when there are more inputs to process or winit has a weird delay.

RenderStage::Cleanup won't work in this case because we need to access the main world to write the time. Seems like we either need to set the time after RenderStage::Cleanup or somehow before winit events start getting processed. Once we have pipelined rendering that could be in the Extract stage.

Feels like it's not very possible to run things before winit events, but if we wanted to, we could fix things a bit in the current model by adding another schedule that is run after sub apps are run.

[alice-i-cecile]

Long and detailed thread on Discord about this issue can be found here. I've done my best to make sure the discussion is reproduced here / in #4728, but if future readers ever want to really dig into the history, that's where it is.

[adsick]

I'm not sure if that's the same, but I get this level of jitter even in simple "move_sprite" example.

hardware: MacBook Pro 2020 (i5)

[superdump]

Long and detailed thread on Discord about this issue can be found here. I've done my best to make sure the discussion is reproduced here / in #4728, but if future readers ever want to really dig into the history, that's where it is.

What is the title of the thread? I use Discord apps and clicking the link doesn't open them on mobile nor desktop.

EDIT: "Timing, stutter, hitching, and jank" in #help.

[inodentry]

Yes, the time jitter is not from the "complexity" of your project, but rather the way Bevy updates its time values. The problem exists even on a minimal blank example app.

[adsick]

bevy/examples/2d on main [?] via ⚙️ v1.62.1 took 4m41s
75% ➜ cargo run --example move_sprite --features wayland --release

I still see sprite moving not smoothly(

Software: Fedora 36 with latest updates
Hardware: Ryzen 5 3500U, Vega 8 graphics, 12Gb 2400MHz dual-channel memory.

[qpshot]

Jitter still exists in simplest 2d sprite moving demo, even with full screen mode.

system：Windows
gpu：RTX3060
cpu：i7-8700k

[smpurkis]

Likewise on my macbook, linux machine and built for the web

[VergilUa]

This issue snowballs badly due to the lack of proper frame lock and while having something like freesync or g-sync.

With low CPU load framerate peaks above monitor refresh rate, then goes lower, then back higher again. Never reaching actual refresh rate. Which causes massive variation of DT and that also results in major stutter.

To have production ready engine this has to be solved, combined with:

• Proper target framerate / lock;
• Physics (that simulates separately and has interpolation or extrapolation);

Right now its really hard to reach smooth gameplay on a decent machine.
If this issue is not possible to solve right now - consider implementing something like a smoothed DT over time.
While it is incorrect and will make simulation framerate dependent, it will at least make the issue less visible for the end users.