Interpolate in GPUParticles makes particles stutter when paused at high FPS.

[MewPurPur]

Godot version

4.0.alpha15

System information

Ubuntu 22.04.1 LTS

Issue description

https://user-images.githubusercontent.com/85438892/188526773-358bc500-d248-4c1d-8f3a-88b9847d4b3f.mp4

With high-speed particles the issue is the most evident.

Steps to reproduce

Create a GPUParticles2D and add a process material, make the particles high-speed, increase project FPS (possibly by turning VSync off) and pause the game.

Some observations:

• At 60 FPS, the issue was non-existent.
• At higher Fixed FPS for the particles the stuttering is reduced, but present nonetheless.

Minimal reproduction project

Didn't have time to make one, but I'll get to it later if it'd help.

[Calinou]

Related to https://github.com/godotengine/godot/issues/50824 and https://github.com/godotengine/godot/issues/51318.

To work around this, disabling Interpolation and setting Fixed Fps to 0 (or a very high value such as 1000) seems to do the trick.

[elvisish]

Related to #50824 and #51318.

To work around this, disabling Interpolation and setting Fixed Fps to 0 (or a very high value such as 1000) seems to do the trick.

This works in beta 10. I can confirm it's still not working beta 10, my refresh rate is 165 and it's jerky (30fps looking) if interpolation is on.

[Mickeon]

I have stumbled upon this issue recently. The workaround looks quite trivial, so it'd be nice to fix this soon instead of having to do apply it for every particle node...

[Calinou]

The workaround looks quite trivial, so it'd be nice to fix this soon instead of having to do apply it for every particle node...

There's a PR changing the default, but reduz would prefer we fix interpolation (which is nontrivial): https://github.com/godotengine/godot/pull/70777

[Mickeon]

Oh, but in my project I temporarily disable interpolation when the GPUParticles node is paused (by checking for NOTIFICATION_PAUSED/UNPAUSED), and that's all. Setting the fixed FPS to 0 doesn't seem to matter to me. Perhaps something has changed after a year?

[Robert-K]

I feel like the engine should just internally disable the interpolation if the particles are paused. They're not gonna move anyways, right?

[Mickeon]

Disabling interpolation on pause would, in theory, make the particles instantly snap to their actual position, which would look jarring. Therefore the solution may not be as straightforward as assumed.

[Robert-K]

Not seeing any jarring jump. It's certainly better than epilepsy introducing jittering ;) For anyone stumbling on this, here's a quick script you can attach to your particles (make sure the 2D/3D matches) to fix the jittering for now:

extends GPUParticles2D

func _notification(what):
    match what:
        NOTIFICATION_PAUSED:
            interpolate = false
        NOTIFICATION_UNPAUSED:
            interpolate = true