Fuel explanation and turning off smoke and docs in general

[davision]

Phoenix produces beautiful results. That is clear. However, I think the documentation is seriously underdeveloped.

It seems to gloss over a lot of important details.

1. Where is the information on "Fuel"? What does it mean exactly? I see you can "Enable burning". Seems to me, if you are simulating fire, then something is always burning. Why does this checkbox make it look different?

I see virtually no information on what Fuel refers to or how it works. Seems like there should be a clear sample included at the very least.

2. How can I disable smoke entirely? If Smoke is set to 0 in the source, it seems like there should be no smoke -- but there is. Is the only way to disable smoke to just turn down the transparency graph?

3. I've enabled Smoke in source, and in Output channels, but the sim appears exactly the same. Why?

It would be nice if the docs had a lot of example images similar to the Vray manual. Two rendered sims should be shown side by side but with one setting changed, so we can understand the visual difference that occurs when adjusting the many abstract parameters. I remember the old Phoenix plugin had these types of comparisons (although it too could have used more) and they save us users a lot of time since we can experiment in a more deliberate and focused way.

Don't get me wrong, Phoenix creates excellent images, but the documentation is inadequate in my opinion.

[Ivaylo Katev]

hi division,
i agree that the "smoke" part produces confusion in many users, and we have to do some work with this in mind. anyway, in brief the concept is as follows:
the simulation and the shading do not have a "hard" connection, the simulation just produce several maps, that you can use in the shading. the shader has a similar concept as the VRayEnvironmentFog, and here comes the first confusion - the default settings do not use the "smoke" map to produce visual smoke, that conflicts with the user's intuition. one can ask then what the "smoke" map does? the answer needs few words about the simulator. so, beside the fluid movement, phoenix is able to simulate some additional processes, like cooling and burning. the burning converts "fuel" into "smoke" releasing certain energy, that heats up the gas and produces certain expansion. like in the real world, the fual can burn only if you have oxygen, but this is not important now. so, you can export the fuel map, the temperature map, and the smoke map, and you can use them in the shading. you are not obligated to shade the "smoke" as smoke and the "temperature" as fire, the shader is flexible. as i said the default settings do not use the physical "smoke", and this leads to confusion in many users that try phoenix for first time. the visual appearance of smoke is produced not by the physical "smoke" result, but by the diffuse color and the opacity. the default shader settings are in physical correct mode - there is no distinct border between fire and smoke, the fire is just smoke that emits light (this is controlled by the emissive curve). to separate them switch on the "ignore alpha" checkbox in the emissive rollout. in this mode the fire is controlled only by the emissive rollout, and the opacity/diffuse rollouts control only the smoke appearance.

regards,
ivo

[davision]

Thank you for the reponse, Ivo.

Perhaps, I understand things a tiny bit better.

But let me ask this... why is there a gap in the default transparency graph -- from 1500 to 1000. Isn't that the transition from fire to smoke? If so, shouldn't the emissive luminance go to 0 within this gap?

Furthermore, I still don't understand 'fuel'. For instance, when should you use it? And when should you not use it?

Can you make one object catch fire from another object? (Is this where fuel comes into play?) I don't see any information in the docs on how this is done.

Also, I'll make a small suggestion -- instead of 'ignore alpha' that switch should have a more informative label -- like "Detach fire from smoke" or "Control fire and smoke separately". Put "ignore alpha" in parentheses if you want.

[Ivaylo Katev]

Isn't that the transition from fire to smoke? If so, shouldn't the emissive luminance go to 0 within this gap?

yes, the left part is responsible for the smoke, the right one for the fire. but there will be fire and smoke even if there is no gap in the curve, it just produces layer-like appearance and illusion for high resolution. the default curve is not the best one, very probably in the next release it will be changed.

Furthermore, I still don't understand 'fuel'. For instance, when should you use it? And when should you not use it?

i think the only reasonable application of the fuel is when you target is a propagating fire (catching fire is a special case of this situation), yes this can be made with animated texture but is harder and less realistic. if you need just a fire, you can made it without fuel.
of course beside this you can use the fuel as a helper channel without any burning, but this is outside of the topic scope.
about the label "ignore alpha" - point taken, really the current caption is not informative and is good to be changed.