Particle systems lagging behind fast moving objects

[wild jackal]

I am trying to use particle systems for the fire FX of vessel thrusters in my game, and I am interpolating their positions in a script to achieve a nice, consistent trail without having to use ps.emission.rateOverDistance which can be finnicky at the extremes (too fast / too slow). I will attach that script below. I will also share some debugging tools I use to help get a frame of reference here.

My thoughts / questions:

• I don't want to predict where the thrusters will be next frame, because this may cause problems with high speed collisions and other similar cases, but if there is an easy & reliable way to do this, I may look into it.
• Does physics running in fixedupdate() vs rendering happening in update() cause problems with physics-enabled objects rendering behind where the truly are?? I don't think this is the case but it would explain this observed behaviour.
• Why do my debug.drawlines appear to confirm that the particle systems do indeed physically lag behind the vessel in world space?

Video of debug & issue in action: https://youtu.be/uGetCXgPTOM (debug lines indicate the difference between the last position of the particle system and the current position)

(Apologies if this should be in #⚛️┃physics or #✨┃vfx-and-particles, but I felt this should be posted here since an in-script solution seems most likely to me)

ignore the discord code block formatting lol I tried to just copy paste first

[wild jackal]

https://paste.mod.gg/fssrqxshvzit/0

btw, Activate(float ratio) is called by controller / brain scripts

[dense stump]

Yes, fixed update can lag behind the regular update. If your vessel is moved in update, then that's the core of the issue.

You either need to love both in fixed update or in normal update.

[wild jackal]

They are both in FixedUpdate(), which is why I am confused

I wonder if this is more specifically an issue with particle systems themselves since it seems like the actual gameobject is lagging behind in space based on the debug information

Not sure how that is even possible when it is patented to the moving object but it’s just a guess

[dense stump]

How is the vessel moved? Is it moved by physics with rb having interpolation on?

[wild jackal]

Yup. Fully physics driven

[dense stump]

If it has interpolation on, then it's moved in update. Or at least interpolated, causing the desync with your particle system.

You should probably move it in update. Is there any reason you need to move it in fixed update?

[wild jackal]

just concerns with framerate affecting the particles' behavior since they can collide with objects & deal fire damage

but this seems like an easier problem to fix, so I may look into moving it in update

[dense stump]

The frame rate is not an issue, you might have physics related issues with it anyway, since you move it via transform:
fireFX.transform.position

If you need it to behave as a physical object, you should add a rigidbody and move it via that in update. Then if you enable interpolation on, it should be in sync with the vessel itself.

[wild jackal]

After working through this for the entirety of my time today, I have decided to ignore this issue unless I am struck by a stroke of genius

[dense stump]

I mean, it should be pretty easy to address. I've already outlined the options you have.

[wild jackal]

I think I did a poor job of outlining all the factors, which is on me