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Sampling Subframes not working 2.8
Open, Confirmed, LowPublic


System Information
Operating system: centos
Graphics card:1080 ti

Blender Version
Broken: 2.8 8b192e1a6a5f

Short description of error
the Sampling Subframes in the flow smoke object setting doent seems to have any effect

Exact steps for others to reproduce the error
Based on the default startup or an attached .blend file (as simple as possible).
c-reate a smoke domaine from a default cube
-create a smoke flow object from a default cube
-create 3 key frame in translation
-bake different simulation in point cache with different Sampling Subframes does not change anything in the result



Event Timeline

Jacques Lucke (JacquesLucke) triaged this task as Needs Information from User priority.

Please provide a simple .blend file that allows us to reproduce the bug more easily.

here is a simpla example from an animated object in blender

Jacques Lucke (JacquesLucke) raised the priority of this task from Needs Information from User to Confirmed, Medium.Feb 12 2019, 3:14 PM
Brecht Van Lommel (brecht) lowered the priority of this task from Confirmed, Medium to Confirmed, Low.Feb 13 2019, 5:16 PM

I need this unbroken now. My smoke flow, in an animation that is important to me, travels extremely fast and leaves completely disjointed smoke puffs in its path. Why did it get set to low priority? Did you think it will be difficult to fix?

Finn Bear (finnb) added a comment.EditedMay 9 2019, 11:52 PM

Looks like this isn't a bug. I found the following in /source/blender/blenkernel/intern/smoke.c line 3027 (after about an hour of searching)

/* Disable substeps for now, since it results in numerical instability */
totalSubsteps = 1.0f;

Does the comment still apply?

If so, how severe is it? Would it be better to cap substeps at a value like 5 or 10, rather than just 1? How long is "for now"?

Also, if you are disabling a feature, shouldn't you do it in the UI and not the kernel? That way, users wouldn't expect it to work.

@Jacques Lucke (JacquesLucke)

Any news on this? I'm also working on a simulation that requires subframe sampling and it would be great for this to work.

You can create a smoke sim in 2.79 and import it I believe. It's worth a try at least. The point data shouldn't care after it's already baked.

You are correct. It is possible to bring it from 2.79. That's what I'm doing now, but it's a much slower (and limited) workflow.

For me, I can't use 2.79 because the smoke flow object was created and edited in 2.8 only. The workflow would be a mess. Wouldn't it be better to remove the "totalSubsteps = 1.0f;" and let people use substeps at their own risk?

Actually, you were right SlyNine. I was able to append the the smoke objects from the 2.8 blend file, bake the simulation in 2.79, and import it back. Still, is this bug/kludge ever going to get fixed?

Hi @Brecht Van Lommel (brecht) and @Jacques Lucke (JacquesLucke)

I know the dev team is busy getting 2.8 ready for launch and I don't mean to nag you guys about this. But I'm a little worried this might have been forgotten. With the release candidate around the corner I would like to see this bug being addressed. As it is, it makes it pretty hard to use the smoke simulator in production with any kind of fast moving objects.

I know you are probably looking to replace the whole smoke simulator with the Mantaflow branch, but if that is not going to happen before the 2.8 release, then this should probably get at least back to how it worked in 2.79.

Would it help to bring this up to @Dalai Felinto (dfelinto) or @Ton Roosendaal (ton) attention? Do you need any more test files?
I'm not a developer, so I can't help much with the code, but I'm happy to help with anything else I can. Just let me know.

This is not going to be fixed for 2.80, sorry, there is no time for it.

Thanks for the quick reply @Brecht Van Lommel (brecht).

I understand we're so close to launch that it's too late to fix.
Would re-enabling the line @Finn Bear (finnb) mentioned be that unstable? (/source/blender/blenkernel/intern/smoke.c line 3027). It seemed to work in 2.79.

If not, I suggest the control being removed. As it is, we have a control in the flow options that doesn't do anything.

Thanks again for the all the great work on 2.8 and I look forward to seeing this bug fixed after the release so I can leave 2.79 behind.

Any plans for this to be fixed before the final since the launch has been delayed with RC2?

Glad to find this report, Having this issue on Mac OS 2.80, and also keen for a fix. Keep up the fine work!