Smoke & fire workflow?

[grimm]

Smoke and clouds are really easy to control and setup, but I'm having all sorts of problems getting smoke + fire working very well. Here is an example of what I have accomplished so far:

To get this I had to crank up absorption and scatter to all white, is this correct? To get the fire looking ok I dialed the power way down and upped the scatter scale, but as you can see I'm starting to get grid artifacts. I'm thinking it's because everything is so cranked up (or down as the case may be)? I checked the file with vdb_view to make sure that there were both density and temperature grids. I even tried to use two volumes, one for smoke and one for fire, but that didn't turn out very good. Is there a method to the madness?

Here is the ocs file:

explosion-test.ocs

(27.06 KiB) Downloaded 237 times

The VDB file is from the OpenVDB website and is called explosion.vdb

[haze]

I haven't had a chance to look at the scene you've attached, but some guidelines for you that might help: Keep a close eye on your Volume step length in your kernel. I suggest you leave the scattering and absorption colours at white, switch to Pathtracing, and set your volume step length to something low, but still manageable by your system. The lower the more accurate it is, but will severely affect performance. Do lower your Scale in your medium. The blocky artefacts you're seeing is caused by an excess in density. Maximise the density, minimise the step length, and the volume will start to behave like a diffuse mesh (which if you think about it, kind of makes sense - a solid cube with a diffuse material is very dense). Hope that helps

[uncia]

explosion_quick_looks.ocs

(29.14 KiB) Downloaded 223 times

Volume_step_1

Volume_step_5

[uncia]

Grid Artifacts free

explosion_no_artifacts.ocs

(27.05 KiB) Downloaded 223 times

[grimm]

I haven't had a chance to look at the scene you've attached, but some guidelines for you that might help: Keep a close eye on your Volume step length in your kernel. I suggest you leave the scattering and absorption colours at white, switch to Pathtracing, and set your volume step length to something low, but still manageable by your system. The lower the more accurate it is, but will severely affect performance. Do lower your Scale in your medium. The blocky artefacts you're seeing is caused by an excess in density. Maximise the density, minimise the step length, and the volume will start to behave like a diffuse mesh (which if you think about it, kind of makes sense - a solid cube with a diffuse material is very dense). Hope that helps

Thanks for looking at this haze, and yes that makes perfect sense.

I used pathtracing and the step length is set to 4m. So to control the fire vs. the smoke I have been using emission texture, emission power, and scale on the volume. This is using the black body emission node. I'm still having a lot of problems getting the look I want. Is there anyway of controlling the smoke and fire separately? Maybe it's a problem with this particular VDB file?

In this image I'm getting closer to what I want, but I have to reverse the inputs from the VDB file, I put temperature in the density slots and density in the temperature slot. In this case emission texture is set to 0.01, emission power is set to 20, and volume scale is set to 5.

So I tried the fire.vdb file to see if it would act different, but it does as well. I can't get the fire and the smoke to separate from each other. On this one I had to bring the emission texture down to 0.00001 or lower which feels a bit extreme to me? The emission power is 50, the volume scale is 4.

Jason

[grimm]

Grid Artifacts free

Thanks uncia, but it's still not quite the look I want, I guess I need a contrast slider?

Here is an example I was thinking of:

https://www.fxguide.com/wp-content/uplo ... losion.jpg

Jason

[haze]

I'm not 100% clear on exactly the settings to reproduce something like the link you posted. I have a feeling it's a careful balance between how the VDB is generated, and the step length, but I can't be sure. I'll take a good look at this when I'm back at work tomorrow.

[Rikk The Gaijin]

The 'smoke and fire' in the above images looks more like a spore, rather that real smoke and fire. I'm a bit worried that haze also doesn't know how to create a realistic one, like the link above, as that's a pretty standard quality for vfx.

[grimm]

I'm not 100% clear on exactly the settings to reproduce something like the link you posted. I have a feeling it's a careful balance between how the VDB is generated, and the step length, but I can't be sure. I'll take a good look at this when I'm back at work tomorrow.

Awesome, thanks haze. I did some more tests with the fire.vdb file. What I have noticed in all of these tests is that if I bump up the fire so it looks like I want it, the emission takes over all of the volume. If I then adjust things to bring the smoke looking like I want, the fire gets clobbered. I need a way to decouple them so they can be adjusted separately.

I reloaded the volume in decimeter scale, so this first test has the volume step set to 0.1, the emission texture to 0.04, emission power to 0.5, and the scale set to 10:

It's looking better but the emission is taking over.

This on I set the volume step to 0.05, the emission texture to 0.04, emission power to 0.5, and the scale is set to 40 to bring the smoke back out.

As you can see now the fire is not right, and the emission is still too much but it has lost it's contrast. Hope this helps.

Jason

[aoktar]

I suppose these vdb files are not very well generated to give your target effect. In last vdb file, smoke and fire channels are not seperated very well. Smoke densities covers the fire densities. So this causes to a decrease on emission effect.