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  • Underwater Fog simulation

    Hey there,

    still adding to my underwater volcano shot, I wanted to create some underwater fog. I started out with vray environment fog, which works ok for low fog near the ground. But since I have quite a lot of height in the scene and I´m missing some general movement in the scene, I wanted to add some very slow moving smoke elements to the scene.

    I started out with the static cloud tutorial and tried to work from there.

    Here is what I´m at right now:

    1. Added turbulence to the simulation with pretty large field.
    2. Animated the emission from frame 0-20 like in the tutorial, not down to zero, but to a much lower value then from the start of emission, to keep more smoke coming over the course of the shot.
    3. Animated the timescale from 1.0 to 0.3 to slow down the movement of the cloud, after the cloud reached a certain stage.

    What I am not happy with yet:

    1. I want the smoke to be dense at the ground and much less dense and much whispier as it is now towards the top. I´m not sure how to get there right now...Should I just lower emission and emit over a much longer period of time with very low dissipation value? Or very low dissipation and stronger buoyancy? Is there a way to get part of
    2. I also want to create more "tentacle" like structures, right now it still looks more like a cloud and less like there are strams of water dissiipating the smoke in all kinds of direction.
    I´m not sure how to achieve that either.

    I´ve thought about:
    A) Creating geometry to emit from, that already kind of resembles the forms I´m after.
    B) Going the realistic route and just adding verious plain forces or wind forces with decay to simulate the water streams or different layers of temperature.
    This might be very R&D intense though to get the look right.

    3. I´m having bit of a hard time understanding the turbulence force. I´m guessing that the pressure mode is more realistic or closer to what an actual windforce would produce, is that correct? Would be great if you could add some examples to the manual for this!
    Even with the force visualisation I´m having a hard time predicting where I´m ending up when changing the parameters.
    I just feel that its easier to control with the standard 3ds max wind force, its always good to have the extra options and control you provide, but are the simulation speeds much different? And is there something to it or am I just imagining the max windforce looking more natural than the Phoenix turbulence?
    i haven´t done too much comparision with these two, its just a feeling right now....

    4. I don´t want the smoke to be too dense overall, while still retaining enough detail, but rendertimes right now are pretty massive (over an hour for the frame attached):
    I´m currently rendering with volumetric geometry, since I´m using the Zbuffer to composite distance fog in After effects.
    But considering that I probably can´t use it with the opacity curves as they are now, I´ll probably switch to volumetric mode and that should already give me a little speed bump.
    Are there any other tips to speed up the rendering?

    I´m also gonna try if I can just render with much higher density, save as 32 bit and then play around with the alpha levels in compositing, to get close to a similar look.

  • #2
    Hey,

    Looking very nice already! I'd suggest you use some smoke dissipation for the fog clouds and just run then on a very low resolution to get the strength and scale of the turbulence right.

    Regarding render time, try turning the phoenix light cache on/off just in case and also experiment with Use Probabilistic Shading and the sample counts in the atmosphere settings.

    Hope this is useful!
    Svetlin Nikolov, Ex Phoenix team lead

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    • #3
      Hey Svetlin, thanks for the input, I´ll play around with these settings. I think I don´t quite understand dissipation right though...If I just increase the dissipation, doesn´t the smoke just also dissapear faster? And if so, can I just add more smoke over time to counter that or do I just have to set the dissipation very low and just run it for a much longer time?

      Probably the best way would still be to try and get those forms with the turbulence first.

      And I am also not clear about the turbulence pressure mode yet...As far as I understand it not, if I just add turbulence without pressure, it kind of just displaces the smoke, while when I´m using pressure mode, it also adds velocity, thus resulting in more turbulence over time. If thats correct, I should probably also animate the turbulence strength so it doesn´t constantly add velocity, right?

      Comment


      • #4
        Hey, yes - if you use dissipation, you need to constantly feed more smoke.

        Both turbulence mode add velocity, but while the regular mode directly creates velocities, pressure mode creates high and low pressure zones and lets the solver naturally create velocities between those, which is more realistic. However, since computer simulations naturally lose velocity with time, unless your turbulence strength is not huge, then the turbulence will reach a stable state. The same goes for example for the Wind force / Plain force - they do constantly add velocity, but it settles to a certain speed after a short while (or explodes )

        Cheers!
        Svetlin Nikolov, Ex Phoenix team lead

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