Slow Motion Smoke woes

[Luchifer]

Hi everyone,

I am currently trying to simulate a powder paint explosion with a speed ramp on it so it has an initial explosion at normal speed, slow down drastically for the middle and then speeds up to normal at the end.

I am trying to follow the instructions on the Chaos Group tips and tricks (on docs.chaosgroup) but I keep running into a wall.

This is my process so far :-

1. I enable Velocity and Wavelet in the Output rollout.
2. I sim the smoke as normal from a TyFlow burst. This sims normally.
3. Once that is done, I go to the input and turn the play speed down to 0.01
4. I go to the resim options, turn on Resim, Grid resim and Time Bend Controls.
5. I press to start the resim and I get an error telling me I have to enable velocity. This was enabled in step one and has not changed.

It only allows me to resim if the play speed is at 1.0 and when I play the resim back it's playing at normal speed, even if I change the play speed to 0.01 in Input AFTER everything has simulated.

What am I doing wrong?

[Svetlin.Nikolov]

Hmm, interesting, are you using Phoenix 3.13?

[Luchifer]

Apparently not. Says I am on 3.11

[Svetlin.Nikolov]

Ah, make sure to grab the latest one - there were some problems exactly with the time bend resim.

[Luchifer]

Just updated Vray and Phoenix. Same process, same result.

"Cannot start the Re-Simulation! Please make sure the simulation was run with the 'Velocity' grid channel checked under the 'Output' rollout, and that the chache files in the Resimulation Input path exist"

I have not done anything different from what the tutorial on Chaos's own site has told me to do.

[Gosho.Genchev]

Just tested on my end - resimulation works for me. Could you please send the file over for us to investigate ?

Cheers!

[Luchifer]

Ok I have sent them to you via WeTransfer.

[Luchifer]

Right, after a few attempts something new has happened. It is now letting me resim but all the smoke is just following particles in straight lines until they reace the edge of the PHX box.

[Gosho.Genchev]

Alright, looks like you've stumbled upon a bug. The Resimulation is trying to read the cache files from the first frame on the timeline (frame 0) instead of the first frame it's being started on (in your case - Simulation roll-out -> Start Frame -> 19).

I'm sure the dev team will look into this as soon as possible. As a workaround, you can change the Time Configuration such that the first frame on the timeline is the first frame of your phoenix simulation. You should also enable the Timeline checkbox on the Simulation rollout.
Works on my end, here's a screenshot with the setup:




Hope that helps!


edit: That's what happens when I'm too quick to write So it's not really a bug - here's what happening:
1. When the Play Speed is reduced to e.g. 0.1 , all the frames from 0 to 200 (from the base 0 to 20 where you're not emitting) either do not exist or are empty because you have no particles there.
2. If you then attempt to start a resimulation, those empty frames will be used which is the reason for the result on the image above. If you didn't even simulate those first 20 frames, everything from 0 to 200 for the resimulation will give you the error 'Missing Velocity'.
3. To resolve this without changing your time configuration, set the Input rollout -> Cache Origin and Timeline Origin parameters to 19 (or whichever is the first frame of the simulation

We tried to illustrate this in the documentation with images, maybe they'll help visualize this: https://docs.chaosgroup.com/display/...meBendControls

[Luchifer]

Thanks for the assist Gosho and I have got further in the resim. It now simulates on the settings you advised but the effect is a bit strange still. As you can see from the photos, the resim has all these streaks coming out as they follow the particles but it's still retaining the cloudy look of the base simulation.

It was set to 0.1 Play Speed, base sim was 11 million cells and I think the resim was something around 95 million.

--Edit-- Ok it seems to be working now. I disabled the particles so they are no longer simulated in with the resim. Now have a lovely slow motion sim. Something to leave running over the weekend.

[Svetlin.Nikolov]

Are you retiming the tyflow particles to 0.1 speed as well?

[Luchifer]

No, if I do that I get a weird backwash sort of effect where the smoke billows around the particles.

[Svetlin.Nikolov]

During resimulation sources continue to emit, so you need to retime the emitter particles, and we need to look at that problem and solve it. How does it look?

[Luchifer]

I got dragged onto something else but I will update with images when I can get back to that project.