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Thread: Realtime fluid dynamics

  1. #1
    CORE 5718 mattclary's Avatar
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    Realtime fluid dynamics

    OK, NASCAR seems to have this, how long before LightWave does?

    http://arstechnica.com/journals/scie...ns-should-care

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    I'm not sure how "realtime" this is...

    Quote Originally Posted by Treuille et al
    We also assume a set of example velocity fields {u_i}, forming a representative basis U of the interactions expected in the system. These example states typically are snapshots drawn from a set of off-line, full-dimensional fluid simulations. We also assume that the velocity fields in U all satisfy two important properties:
    1. The states are divergence free: nabla · u_i = 0.
    2. The states satisfy free-slip boundary conditions: for all fixed surface points x with normal n, we have u (x) · n = 0.
    http://grail.cs.washington.edu/proje...1-treuille.pdf

    Enforcing mass conservation (property 1 above, nabla dot u_i = 0) is actually one of the most difficult (time consuming) steps, so if the offline data is already divergence free, I'm guessing the new sim data can become so in much fewer iterations... but if the offline data isnt there....hmm.

    Granted, I've only skimmed the paper, and Treuille et al ARE a very capable bunch of folks (I think he was actually involved with a keyframe controlled simulation of smoke and water--very cool stuff!!), so I wouldn't be surprised if this is some pretty revolutionary stuff...

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    Working Monkey StereoMike's Avatar
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    Watching a sample on the espn page I had the feeling it loops. So my guess is, they parent a pre-computed particle flow to a car. The graphics card has only to do particle translation (and rotation) and keep it in place.
    I say fake.

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    Heir to the Throne RedBull's Avatar
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    Realtime is exactly that a shortcut way of faking things, so i don't want game level CFD, while a low res high interactivity is desired it's unlikely to get realtime CFD of any worth anytime soon..

    Meanwhile check out Blenders upcoming new "Magic Fluids" (technically it's already built in, but has not been activated in the release version)

    http://www10.informatik.uni-erlangen...ithue/ani_mfc/

    Also if your interested in the Siggraph papers in particular the Fluids and Physical Simulation topics covered on Thursday at Siggraph for 07, there is some good stuff to read and watch in regards to these topics here...
    http://trowley.org/sig2007.html (as there is every year)

    I notice that CSIRO which is our national science research labs, (just down the road) is making a Maya plugin for the fluids paper given at Siggraph.

    Also check out www.cfd.com
    which has just about every known CFD software simulator listed..

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    Quote Originally Posted by RedBull
    Meanwhile check out Blenders upcoming new "Magic Fluids" (technically it's already built in, but has not been activated in the release version)

    http://www10.informatik.uni-erlangen...ithue/ani_mfc/
    Yeah, but its still LBM, right? Personally, I've never been fond of the behavior/appearance of LBM...but thats probably just me.

  6. #6
    maya has nr realtime fluid/particle dynamics i remember alias chap who made it also wrote a 2d version for his palm pc way back ''just for fun''...not had the opportunity to look into maya's unlimited's capabilities myself mind you.
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    Quote Originally Posted by cresshead
    maya has nr realtime fluid/particle dynamics i remember alias chap who made it also wrote a 2d version for his palm pc way back ''just for fun''...not had the opportunity to look into maya's unlimited's capabilities myself mind you.
    jos stam?

    he actually released the source in one of his papers... I made it into a realtime 2D gas java applet once, which was fun.

  8. #8
    that's the chappy!
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    Dazed and confused lilrayray77's Avatar
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    Quote Originally Posted by RedBull
    Meanwhile check out Blenders upcoming new "Magic Fluids" (technically it's already built in, but has not been activated in the release version)

    http://www10.informatik.uni-erlangen...ithue/ani_mfc/
    Looks interesting, although, from the example, the water seemed rather viscous, but my eyes could be deceiving me. All the more reason to start learning that trademark blender interface ...
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    Quote Originally Posted by lilrayray77
    Looks interesting, although, from the example, the water seemed rather viscous, but my eyes could be deceiving me. All the more reason to start learning that trademark blender interface ...
    That's a result of the simulation method they use, Lattice-Boltzmann (LBM). IMHO, it looks goopier and not as good as the other methods. Also, lower viscosities take longer to simulate using LBM, so that could contribute...

  11. #11
    Ach, Mein thirsten! Anti-Distinctly's Avatar
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    Quote Originally Posted by loki74
    That's a result of the simulation method they use, Lattice-Boltzmann (LBM). IMHO, it looks goopier and not as good as the other methods. Also, lower viscosities take longer to simulate using LBM, so that could contribute...
    It could also be the surface recontruction that they use. The isosurface that is reconstructed looks as though it has the same resolution as the LBM grid, implying that it just uses a marching cubes style algorithm only. To get proper real nice Fedkiw style surafaces you ahve to use the level set technique.
    This is all supposition of course
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    Quote Originally Posted by Anti-Distinctly
    It could also be the surface recontruction that they use. The isosurface that is reconstructed looks as though it has the same resolution as the LBM grid, implying that it just uses a marching cubes style algorithm only. To get proper real nice Fedkiw style surafaces you ahve to use the level set technique.
    This is all supposition of course
    Ah, level set is actually a volumetric signed distance field--it still has got to be triangluated with marching cubes (or some other meshing method).

    But you make an excellent point--using a lower or equal-to resolution with marching cubes compared to the simulation grid could contribute to the blobbiness!

  13. #13
    Ach, Mein thirsten! Anti-Distinctly's Avatar
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    Quote Originally Posted by loki74
    Ah, level set is actually a volumetric signed distance field--it still has got to be triangluated with marching cubes (or some other meshing method).
    Totally true, but with marching cubes only you're resolution is limited by the resolution of your LBM grid (I think, I'm not too hot on the LBM), but if you use the results of the LBM on a level set, you can have that evolve on a grid of much higher resolution.
    Again, I'm just making assumptions, but the fluids in Blender I thought was someone's implementation of the LBM. Not sure how concerned they were with the surface extaction. Level sets can be a ***** apparantly too, so marching cubes would've saved a lot of headaches
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  14. #14
    Heir to the Throne RedBull's Avatar
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    LBM does suffer in memory consumption, but personally i prefer it to the NS solvers I've used in applications like Realflow and Maya.

    To remove the blobbyness which i don't personally mind too much anyway.
    you need to use a large resolution. (I Believe Glu3D used LBM too, and i also like it over standard Maya)

    I do have the optimized AMD SSE3 64bit version of Blender, so i must say over standard Blender release I'm able to render larger quality now and much faster speeds.

    And for this reason it's fairly good to me, and it took a day to master...
    (it's just that awkward Blender interface.. I seem to get much faster calcs out of Blender than Reaflow, and Blender isn't multithreaded for it's calcs.

    Houdini finally has the old http://www.myrtlesoftware.com/ fluids solver built in 9.0, (also note that Maya has Myrtles 5x5 Voxel Renderer coming, (goodbye Hypervoxels) And they are rewriting Digital Domains "Storm" Renderer..
    I'm getting jealous of the FX of others.

    I have not tried the Houdini one, although i have loaded the Beta. Don't know what algorithm they use it that one... (I Believe it's N-S)
    I believe the CSIRO are making a SPH plugin for Maya.

    Also my previous link for CFD link was incorrect, actual site is this: http://www.cfd-online.com/ (check out the Links link for software)

    As for some more Realtime stuff, that was originally the link i meant to post
    I don't know why i posted the magic fluids link... As it was a little old anyway.

    http://www.cemyuksel.com/research/waveparticles/

    Some of the boats and boxes look good for realtime, but the way the edges of waves interact with sides of some of the tanks, doesn't convince me...
    But still cool to look at...
    Last edited by RedBull; 08-12-2007 at 06:45 PM.

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    Uh, wouldn't RF use some form of SPH? Unless its solving a NSE velo field and simply advecting particles around in that....

    Anyway... I guess we'll just have to agree to disagree on this one. I have never in my life, at any resolution, seen LBM that didn't look somewhat viscous. The fact is, most of the comlex liquid behavior (as far as basic liquids, so excluding viscoelasticity, combustion, foaming, melting, etc) is in the small scale turbulence observed in very low viscosity liquids (ie, water), as well as the breaking wave situation.

    The fact is, LBM sucks at both low viscosity high turbulence simulations, and I've yet to see a breaking wave simulation with LBM.

    You've already mentioned the RAM problem, but at super high resolutions, this does become a serious problem. And, as you've stated, high resolution is needed for very good results.

    But all in all, by BIGGEST beef is that while neither NSE or LBM are very easy to implement, the concepts behind NSE are MUCH easier to understand. Even with Mr. Thurey's psudeocode, I can't seem to wrap my mind around LBM. NSE on the other hand, is a very clear process, as Stam proved in his 100-line realtime sim code.

    =====

    Anti-Distictly--I don't completley get LBM... are you referring to using LBM to calculate velocities, but actually tracking the fluid on a LS grid? Up resing the LS grid compared to the velo grid can be nice, but you will end up with disconnected pieces of fluid affecting each other's motion without contact as a result (this happened in Gokteken et al's viscoelasticity paper...)

    But either way, I think its less of an issue if you just use eulerian NSE from the start--that way you'll be using a level set anyway! (~_^)

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