Smoke rendering and lightning in closed rooms

[ben_hamburg]

Hi there,

I´m back with another observation. I´ve recently worked on some smoke underwater and I had some trouble making the smoke look as dense as in other examples.
Now I have another shot (not underwater) with a similar problem and I accidentally made an observation, I already had suspected before: The lighting does indeed play a huge role in the overall look.
I attached two pictures to highlight this:

In the first picture the lighting is the same, but its inside a closed container (the tube).



In the second I accidentally rendered without the tube and I got a much more "volumetric" look.



There are three spherical Vray lights in this scene: One coming from above the smoke emitter inside the tube, one coming from the front of the smoke and one placed outside the tube for the yellow glowing look.
So now I´m wondering if there is some option in the rendering settings that I´m missing or if I´m just beeing dumb, since this is just how the actual lighting inside the tube would make the smoke look like.
I also didn´t like the look of he smoke rendered with the tube around it, because it didn´t seem to integrate well with its surrounding.
I´m currently doing another sim, so I can´t test some things I had in mind (like excluding the tube from casting shadows), I´ll post my findings but would also appreciate some input from the professionals...

[ben_hamburg]

You should definitely put some examples in the manual of how to use the opacity curves and the smoke colors...I still find it incredibly fascinating how many different looks you can get from the same simulatinon data and how it can go from very thin, cigarette-smoke looking to nuclear war with just some adjustments to the opacity curves...Right now I´m trying to find the sweet spot between where the smoke gets the "nuclear"-volumetric look and the more single colored but more transparent "cigarette-smoke", I always seem to get either the one or the other.
And again: This might just the way its physically meant to be, but art direction always (in this case my own...) always wants different stuff than whats "realistic"....

[Svetlin.Nikolov]

Hey,

Not really sure what to suggest here - when the smoke is in the pipe, you could try changing the scattering from approximate to disabled - this would make it darker and would probably make it blend better with the pipe. GI will also help make the transition more natural, but will take some time to render.

Otherwise - mastering the fire and opacity curves is true art. Really have to explore ways to make it more straightforward and predictable...

Please share how this goes

Cheers!

[Neilg]

I find the opacity curves have too many points by default and it takes a long time to edit and figure out where the basic ends of it are.

If there was something like the preview temp controls where you can set a minimum and a maximum value with viewport feedback, then click a button that draws a straight line going from one value to the other that we can start adding points to it, i think that would help a lot. just some way to reset it to an arbitrary scale that we've got control over would be good.

[squintnic]

Agree.
Opacity curves are a bit of a nightmare for me.
Is there was a practical example of how to best set them up for different looks?
Thanks

[Svetlin.Nikolov]

Thanks guys! Please do share anything specific that bothers you - we have to make those as usable and easy as possible.

@Neilg, we reduced the default points from about 20 to 5, but since the fire opacity curve is no longer logarithmic, we can drop this to just 2. In the nightlies, and the VolumeGrid we added that cyan-colored zone which denoted where the data interval of the channel is - do you find this useful? I think we should extend it by encoding a histogram of the data into it - where there's more voxels of a certain value, the zone will be more opaque, and vice versa. This way you'd know where there is more concentrated data that will affect the rendering more.

@squintnic, I agree, we should add more such presets to the Presets dropdown in the Rendering panel. Right now I can think of several such additional presets - there could be a fire curve that makes the colder parts shine brighter and this reverse effects makes flames looks quite hot and dynamic. Also there could be several example curves for smoke - a single or double spike curve that can be used for cigarette and coffee smoke, and an S-shaped curve that can be used to make smoke appear more massive. Do you have other examples of smoke or fire looks that we should build in?

Cheers!

[ben_hamburg]

Yeah, I tried GI and apart from the hefty increase in renderingtimes it also looked a bit weird, I think I tried both IR+LC and BF+LC.
The first one produced results that looked just wrong, like badly interpolated GI, the second also looked weird, I didn´t save it out, so I can´t post a screenshot right now.
It did blend better though.
For now I just played with the opacity curves again, changed the lighting (excluding the pipe from shadowcasting really made the smoke "pop") and then just did some color correction in post.


Could still do some more work on it, but for this shots intention it worked pretty well.

I´m just really happy to figure out how much the lighting has to do with the look, because it was driving me nuts, that I couldn´t achieve the same look as in the examples.

I don´t think the opacity curves are incredibly hard to master, its more about the overall understanding of the software. I´m still at the point, where I´m not sure how all of the parameters come together to achieve a certain outcome, so I´m always "tweaking all knobs at the same time", so to speak. I mostly don´t have time to do enough R&D to find out which parameter does what exactly, thats why I´m very depending on the manual. Whenever a paramter is being explained with some visual examples, it´s just so much easier to understand. Thats why I think you should try and include some examples on the transparency and gradient tabs in the manual, the explanation is kind of very general there and makes it hard to understand the impact it can have on the final result. For example: It might not matter very much, how much smoke I emit or how strong the gravity in my scene is-I migt just have to increase the opaquenes off the smoke to make it look more massive...

[abri]

Thanks guys! Please do share anything specific that bothers you - we have to make those as usable and easy as possible.

Do you have other examples of smoke or fire looks that we should build in?

Cheers!

Hi Nikolov,

I for instance have/had a hard time setting up the illumination, density and opaqueness for such a simulation:
https://www.youtube.com/watch?v=g5U-ZUKlLP4

[Svetlin.Nikolov]

Hey,

Could you share the result you achieved and what settings you used? The render settings should be close to defaults, except a blue emission color and a smoke opacity curve with a single or double spike.

Cheers!

[abri]

I would appreciate a demo scene for following example:

btw, 200 frames take just about 10 seconds to be created in fumefx..

[flipbook]

do you have nightly access? I'm sure that is one of the presets, not 100% sure tho.

[Svetlin.Nikolov]

That would be a preset that simulates only the liquid.

We are starting to repeat things though - as noted a couple of posts back, cigarette and coffee smoke are easily achievable with a single or a double spike in the opacity diagram, using the smoke as base. @abri, have you tried that?

[abri]

That would be a preset that simulates only the liquid.

We are starting to repeat things though - as noted a couple of posts back, cigarette and coffee smoke are easily achievable with a single or a double spike in the opacity diagram, using the smoke as base. @abri, have you tried that?

Therefore I was only asking for a simple demo scene.
Nevertheless, the thin smoke looks different (I cannot create smooth fade outs on the borders) and entering data in the opacity dialog is not easy.
Here is one of my results. Look at the very small values I have to use..

[Svetlin.Nikolov]

Yop, if you want to make the borders smoother, you have to make the spike wider. Also, note that moving the spike left and right might give a better look to it. Regarding the high opacity, could it be that the scene scale is too high? By default Phoenix scales the opacity according to unit scale, like the VRayMtl fog color. Phoenix v3.0 will have an option to turn this off, but it's still not in the nightlies, so you might have to use very low opacity values.

Also, a completely different approach would be the one in the "ink in water" sample scene (you can find it in your Documents folder or at the docs site). In this example scene, drag particles are created instead of smoke and are shaded as points - this way you'll have a similar effect, but not quite the same - the Smoke opacity spike will always form a closed surface, while the particle approach can form all kinds of different shapes.

Cheers!

[abri]

Hi Nikolov,
concerning the scene scale, should then this scene not look different?


Moving the spike around without changing the Y value is not so easy..

I know the "ink in water" example but a) it takes hours to simulate an b) I did not find a way on how to set buoyancy.