Mantaflow 'Dissolve' broken for Flame smoke #74192

New Issue

Andrew Price · 2020-02-25T09:30:20+01:00

Andrew Price commented

2020-02-25 09:30:20 +01:00

System Information
Operating system: Win 10 64bit
Graphics card: Dual Titan RTX

Blender Version
Broken: 2.83 2020-02-24 a322b43e3d

Short description of error
Enabling 'Dissolve' in Mantaflow Gas with any emitter producing fire, will completely disable the smoke normally produced by the fire.

Exact steps for others to reproduce the error

Set default cube to a Fluid>Gas>Domain
Create a plane inside. Set to Fluid>Flow, and set the flow type to Fire.
In the domain settings, under Smoke set the Dissolve to 150.
The fire will render, but there will be zero smoke.
Download Attached Blend: bug_dissolve.blend

The correct functionality would be for the smoke to dissolve from the time of it's birth - not of the emitter, but when the flame created it.

This is crucial to achieving realistic explosions, as the smoke looks patchy due to the varied times it was created from the flames.

Reference video of real explosion. Note how the smoke looks patchy towards the end:
Gas_Explosion_7_1920.mp4

**System Information** Operating system: Win 10 64bit Graphics card: Dual Titan RTX **Blender Version** Broken: 2.83 2020-02-24 a322b43e3d08 **Short description of error** Enabling 'Dissolve' in Mantaflow Gas with any emitter producing fire, will completely disable the smoke normally produced by the fire. **Exact steps for others to reproduce the error** Set default cube to a Fluid>Gas>Domain Create a plane inside. Set to Fluid>Flow, and set the flow type to Fire. In the domain settings, under Smoke set the Dissolve to 150. The fire will render, but there will be zero smoke. Download Attached Blend: [bug_dissolve.blend](https://archive.blender.org/developer/F8368881/bug_dissolve.blend) ![dissolve on.png](https://archive.blender.org/developer/F8368854/dissolve_on.png) ![dissolve off.png](https://archive.blender.org/developer/F8368857/dissolve_off.png) The correct functionality would be for the smoke to dissolve from the time of it's birth - not of the emitter, but when the flame created it. This is crucial to achieving realistic explosions, as the smoke looks patchy due to the varied times it was created from the flames. Reference video of real explosion. Note how the smoke looks patchy towards the end: [Gas_Explosion_7_1920.mp4](https://archive.blender.org/developer/F8370481/Gas_Explosion_7_1920.mp4)

Andrew Price commented

2020-02-25 09:30:20 +01:00

Added subscriber: @AndrewPrice

Vitor Boschi commented

2020-02-26 12:39:35 +01:00

Added subscriber: @vitorboschi

Richard Antalik commented

2020-02-28 18:09:43 +01:00

Added subscribers: @sebbas, @iss

Richard Antalik commented

2020-02-28 18:09:43 +01:00

Seems indeed wrong to me, but I don't have clear image how this is supposed to work.
@sebbas can you look at this?

Seems indeed wrong to me, but I don't have clear image how this is supposed to work. @sebbas can you look at this?

Christian Beckers commented

2020-03-12 01:59:42 +01:00

Added subscriber: @BeckersC

Sebastián Barschkis was assigned by Gottfried Hofmann

2020-03-16 15:59:32 +01:00

Gottfried Hofmann commented

2020-03-16 15:59:32 +01:00

Changed status from 'Needs Triage' to: 'Confirmed'

Gottfried Hofmann commented

2020-03-16 16:01:37 +01:00

Added subscriber: @GottfriedHofmann

Gottfried Hofmann commented

2020-03-16 16:01:37 +01:00

Confirmed. There is absolutely no smoke when turning on "Dissolve". But it returns when enabling "Slow".

Note: Tested on blender-2.83-f06a6e92bc5e-linux64 (today's build). The file also crashes with segfault when turning on material preview shading from the layout workspace once it has baked.

Confirmed. There is absolutely no smoke when turning on "Dissolve". But it returns when enabling "Slow". Note: Tested on blender-2.83-f06a6e92bc5e-linux64 (today's build). The file also crashes with segfault when turning on material preview shading from the layout workspace once it has baked.

Lord Odin commented

2020-03-18 14:03:09 +01:00

Added subscriber: @derekbarker

Lord Odin commented

2020-03-18 14:03:09 +01:00

I too can confirm this is happening in my scenes as well

Sebastián Barschkis commented

2020-03-18 23:03:51 +01:00

Changed status from 'Confirmed' to: 'Needs User Info'

Sebastián Barschkis commented

2020-03-18 23:03:51 +01:00

I think the tooltip was a bit misleading. It has been corrected in efb53f5181.

Regarding the fire example, @AndrewPrice can you try enabling the "Slow" option. Or increase the "Time" value even more.

I think the tooltip was a bit misleading. It has been corrected in efb53f5181a7. Regarding the fire example, @AndrewPrice can you try enabling the "Slow" option. Or increase the "Time" value even more.

Andrew Price commented

2020-03-19 04:23:39 +01:00

You're right. Seems if you increase it to 5000 the smoke is visible for a few frames after fire disperses:
dissolve 5000.mp4

It's good that it works, but it's wildly different to Smoke Only, where even a value of 50 creates much longer dissolve times.

And if you enable Slow, it complicates it further by acting on yet another unit of time (seems to almost multiply the current slow value?).

So it seems the bug isn't that Dissolve doesn't work for fire, it's that there is no standardized units. Users might only discover the right values by running multiple tests.

You're right. Seems if you increase it to *5000* the smoke is visible for a few frames after fire disperses: [dissolve 5000.mp4](https://archive.blender.org/developer/F8415011/dissolve_5000.mp4) It's good that it works, but it's wildly different to **Smoke Only**, where even a value of *50* creates much longer dissolve times. And if you **enable Slow**, it complicates it further by acting on yet another unit of time (seems to almost multiply the current slow value?). So it seems the bug isn't that Dissolve doesn't work for fire, it's that there is no standardized units. Users might only discover the right values by running multiple tests.

Sebastián Barschkis commented

2020-03-19 19:29:01 +01:00

I'd say the reason it takes longer to dissolve smoke from a "Smoke Only" emitter is that it produces a lot more density, i.e. their density values are a lot higher. The "Fire Only" emitter might just produce densities that are too small.

Setting the "Flame Smoke" to a higher value should at least give more smoke that results from fire/fuel. Note that one can enter a higher value "manually" (max value is 8.0 right now).

For the "Slow" option, why is it not helpful? The idea is that the smoke dissolves in a 1/x fashion (fast at first, then slower).

I'd say the reason it takes longer to dissolve smoke from a "Smoke Only" emitter is that it produces a lot more density, i.e. their density values are a lot higher. The "Fire Only" emitter might just produce densities that are too small. Setting the "Flame Smoke" to a higher value should at least give more smoke that results from fire/fuel. Note that one can enter a higher value "manually" (max value is 8.0 right now). For the "Slow" option, why is it not helpful? The idea is that the smoke dissolves in a 1/x fashion (fast at first, then slower).

Andrew Price commented

2020-03-20 02:08:02 +01:00

Added subscriber: @Blendify

Andrew Price commented

2020-03-20 02:08:02 +01:00

@sebbas The Slow option is confusing because the user expects it to change the weight of the dissolve (similar to a power curve), but not the end frame.
dissolve slow on off.mp4

But in the above, you can see that Dissolve previously expired the smoke at frame 35, but with Slow Enabled now the dissolve time expiration is something arbitrary. So if the user was previously happy with the time it expired, now they need to re-bake it again to find the new dissolve value for the same expired frame.
Ultimate it comes down to usability. Simulations already require many many bakes (I've already done about 300) to make something they like. Changing Slow to only change the existing behaviour (a power curve, not the expiration frame), it would mean one less thing for the user to test.

As for the dissolve functionality: thank you! Finally I understand this mystery. Though without documentation I doubt anyone will discover how to use dissolve with fire because Flame Smoke's default of 1 is so different to the smoke's default of 1 (which is far, far greater).

@Blendify I think this should probably be added to the Dissolve documentation. A suggestion:
"Dissolve is heavily dependent on the density of the smoke produced. Smoke Onlyemitters typically produces more smoke and therefore a value of 50 might be suitable. But Fire only emitters typically produce less smoke, and therefore may require higher dissolve values of around 5,000-10,000 for the same appearance."

@sebbas The **Slow** option is confusing because the user expects it to change the weight of the dissolve (similar to a power curve), but not the end frame. [dissolve slow on off.mp4](https://archive.blender.org/developer/F8416799/dissolve_slow_on_off.mp4) But in the above, you can see that Dissolve previously expired the smoke at frame 35, but with Slow Enabled now the dissolve time expiration is something arbitrary. So if the user was previously happy with the time it expired, now they need to re-bake it again to find the new dissolve value for the same expired frame. Ultimate it comes down to usability. Simulations already require many many bakes (I've already done about 300) to make something they like. Changing Slow to only change the existing behaviour (a power curve, not the expiration frame), it would mean one less thing for the user to test. As for the dissolve functionality: thank you! Finally I understand this mystery. Though without documentation I doubt anyone will discover how to use dissolve with fire because Flame Smoke's default of 1 is so different to the smoke's default of 1 (which is far, far greater). @Blendify I think this should probably be added to the Dissolve documentation. A suggestion: "Dissolve is heavily dependent on the density of the smoke produced. *Smoke Only*emitters typically produces more smoke and therefore a value of 50 might be suitable. But *Fire only* emitters typically produce less smoke, and therefore may require higher dissolve values of around 5,000-10,000 for the same appearance."

Gottfried Hofmann commented

2020-03-20 09:10:53 +01:00

In #74192#894703, @sebbas wrote:
I'd say the reason it takes longer to dissolve smoke from a "Smoke Only" emitter is that it produces a lot more density, i.e. their density values are a lot higher. The "Fire Only" emitter might just produce densities that are too small.

Setting the "Flame Smoke" to a higher value should at least give more smoke that results from fire/fuel. Note that one can enter a higher value "manually" (max value is 8.0 right now).

For the "Slow" option, why is it not helpful? The idea is that the smoke dissolves in a 1/x fashion (fast at first, then slower).

The current behaviour seems like a bug to me.
In 2.79b turning on "Dissolve" without slow does not kill the smoke entirely, it just dissolves rapidly. Even at the aggressive default value of 5.

In 2.83 "Dissolve" just kills the smoke entirely, even on the frame it is emitted. Even at a value of 50. "Slow" makes it come back, even at a value of 5. Here are demo images showing only the "Density" component using an emission shader:

Here is the file: quicksmoke283.blend

Regarding changing the behaviour of the Slow option - we could just try it and see what users like better. It would still change the way the smoke dissolves and thus could trigger the need for rebakes...

> In #74192#894703, @sebbas wrote: > I'd say the reason it takes longer to dissolve smoke from a "Smoke Only" emitter is that it produces a lot more density, i.e. their density values are a lot higher. The "Fire Only" emitter might just produce densities that are too small. > > Setting the "Flame Smoke" to a higher value should at least give more smoke that results from fire/fuel. Note that one can enter a higher value "manually" (max value is 8.0 right now). > > For the "Slow" option, why is it not helpful? The idea is that the smoke dissolves in a 1/x fashion (fast at first, then slower). The current behaviour seems like a bug to me. In 2.79b turning on "Dissolve" without slow does not kill the smoke entirely, it just dissolves rapidly. Even at the aggressive default value of 5. In 2.83 "Dissolve" just kills the smoke entirely, even on the frame it is emitted. Even at a value of 50. "Slow" makes it come back, even at a value of 5. Here are demo images showing only the "Density" component using an emission shader: ![image.png](https://archive.blender.org/developer/F8417267/image.png) ![image.png](https://archive.blender.org/developer/F8417270/image.png) ![image.png](https://archive.blender.org/developer/F8417272/image.png) Here is the file: [quicksmoke283.blend](https://archive.blender.org/developer/F8417276/quicksmoke283.blend) Regarding changing the behaviour of the Slow option - we could just try it and see what users like better. It would still change the way the smoke dissolves and thus could trigger the need for rebakes...

Philipp Oeser commented

2020-04-03 17:57:53 +02:00

Added subscriber: @lichtwerk

Philipp Oeser commented

2020-04-03 17:57:53 +02:00

Changed status from 'Needs User Info' to: 'Needs Triage'

Philipp Oeser commented

2020-04-03 17:57:53 +02:00

Not entirely sure from scanning over the comments, but it seems this should be getting another look @sebbas?

Jakub Grodzki commented

2020-07-05 13:11:35 +02:00

Added subscriber: @kubagrodzki

Jakub Grodzki commented

2020-07-05 13:11:35 +02:00

I agree with @AndrewPrice that something is wrong with Dissolve parameter. My generated by fire smoke disappears suddenly for eg. at frame 20 of my simulation, even though in previous one nearly half of domain was full of it.

Iago Mota commented

2020-09-20 15:12:19 +02:00

Added subscriber: @Iago-Diogo-de-Vasconcelos-Mota

Philipp Oeser commented

2020-11-17 15:56:01 +01:00

Changed status from 'Needs Triage' to: 'Needs Developer To Reproduce'

Philipp Oeser commented

2020-11-17 15:56:01 +01:00

Think there is not much the triaging team can do, waiting for @sebbas

Jakub Grodzki commented

2021-03-12 10:02:30 +01:00

@lichtwerk @sebbas Any updates on this one?

Sander commented

2022-10-14 20:24:31 +02:00

Added subscriber: @Sworly

Sander commented

2022-10-14 20:24:31 +02:00

I'm pretty sure this bug still exists in 3.3.1: enabling "Dissolve" without "slow" will end the entire simulation after a while, instead if killing individual particles after the Time parameter.

Philipp Oeser removed the

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Mantaflow 'Dissolve' broken for Flame smoke #74192