Introduction to Position Based Fluids

http://physxinfo.com/news/11109/introduction-to-position-based-fluids/

by choult • 13 years ago

273 77 13 years ago

_crumbs• 13 years ago

The clip showing the water interacting with breaking glass blew my mind. It may well be a while before we see anything close to that level of realism being incorporated into games though.

Cthulhu_• 13 years ago

True, but technologies like this usually make their way into pre-rendering first, i.e. pre-rendered cutscenes in games, CGI scenes in movies, or fullly animated movies. Although the latter category hasn't seen much in the way of realism, mostly due to uncanny valleyness.

MrScruff• 13 years ago

This isn't prerendered - it's real time. Prerendered water (as seen in movies) looks far more realistic than this. Check out the Naiad link someone posted above.

This is great for real time though. It's using a form of SPH which is far easier to adapt for GPUs, and can crunch through those particle-particle interactions.

High end offline water sims would use a different algorithm (usually FLIP), maybe run for several days to sim a few seconds and occupy 10's of Gb of memory.

If anyone's interested, download a copy of Houdini Apprentice for free and have a play.

rangibaby2• 13 years ago

This is pretty much state-of-the-art game water (as of 2013):

http://youtu.be/65slGvDLmIE

kevingadd2• 13 years ago

I disagree. Games like Uncharted 3 have had much more sophisticated water simulations where water is an active part of the environment. Another example would be Hydrophobia. Crysis 3 is pretty much just slapping a heightmap onto a water surface, but it's no surprise that they take the easy route there since water is not an integral part of the experience in anyway. Crysis 3 is about shootin' men and robots that look like men, not about interacting with physically simulated water.

Here are slides from a presentation about Uncharted 3's water:

http://twvideo01.ubm-us.net/o1/vault/gdc2012/slides/1_Visual...

I believe they've talked elsewhere about the complexity of the water sims they did for areas that involved boats and an ocean you could swim through, but I don't have a link handy.

Y0L02• 13 years ago

Particle generators like this have been used in Hollywood for quite awhile, I bet you would have never have guessed it was CGI though.

atdrummond• 13 years ago

The difference being that this demonstration is real-time.

Y0L02• 13 years ago

Thank you for clearing that up, now this is actually impressive, although I would imagine it would require a ton of GPU & CPU power.

incision• 13 years ago

Perhaps it is the most realistic, but I've yet to see a fluid demo, this one included that didn't look like overly "energetic" shampoo.

Scriptor• 13 years ago

It's definitely realistic, but sometimes it seems just a little off. There are times when it seems to "clump" up like a solid would when you have water particles thrown in the air. I'm not sure if that's my own bias or the uncanny valley effect at play here.

rmrfrmrf• 13 years ago

I noticed that too; seems like it might be a matter of making the particles smaller.

th0ma5• 13 years ago

When I look at this water, it feels as though the volume of water is the size of like an acre. Is this to exaggerate the effect or is this a limitation of our current modeling capability?

carlob2• 13 years ago

Yeah, what you're referring to is the fact that surface tension is not scale invariant. In other words: dew can only be made of smallish droplets, not large ones.

I think the reason you see that is that they don't really simulate surface tension, but use a rather crude trick [0]. My uneducated guess is that in oder to achieve water that looks ok at small scales they would have to increase the size of the particles to the point it doesn't look smooth anymore.

[0] equation (14) of http://mmacklin.com/pbf_sig_preprint.pdf

wfunction2• 13 years ago

> Yeah, what you're referring to is the fact that surface tension is not scale invariant. In other words: dew can only be made of smallish droplets, not large ones.

You completely blew my mind there! I'd never noticed that before.

Does the size of a dew drop relate to the size of the particles? i.e. can we estimate the size of water molecules just by looking at the maximum dew drop size?

simonsquiff• 13 years ago

If you have an iPad I highly recommend getting LiquidSketch. It's a great puzzler about getting pools of liquid into the right place, and the liquid effect is stunning. It moves exactly as expected, has surface tension, splits and recombines etc - but the really stunning element is colour mixing. When two groups of different colours combine the mixing of them is just utter convincing, and beautiful. Amazing how this was all done on a lowly iPad.

MikeCapone• 13 years ago

http://www.liquidsketch.com/

There's a demo here:

http://www.youtube.com/watch?v=dpNjEEtodtg

vilya• 13 years ago

When you say "on a lowly iPad", bear in mind that the latest (4th-gen) iPad has a dual core 1.4 GHz chip plus a GPU capable of 14.4 GFLOPs. It's actually a pretty powerful machine.

miahi2• 13 years ago

You still cannot compare it with the GPU from the OP rendering, that does 3000 GFLOPS (GTX 680).

stephencanon• 13 years ago

The CPU pulls tens of GFLOPs in single precision, too (and > 5 GFLOPs double precision!) It’s at least as powerful as any desktop you are likely to have had a decade ago.

Tyrant505• 13 years ago

Now I finally wish I had an iPad. Waiting on the mini retina...I guess

aeontech• 13 years ago

I had the same concern, but after having a mini for a while, I can tell you that I don't notice the lack of retina resolution at all. They hold their value really well too - I highly recommend just getting one now and selling it once retina comes out.

anigbrowl• 13 years ago

What a charming little game. Just watching the video was strangely relaxing.

coherentpony5• 13 years ago

Jaw-dropping? Maybe. Realistic? Nope.

I work in a group with scientists whose sole job is to solve the Navier-Stokes equations. Now that's realistic.

pvnick• 13 years ago

You must just be a hoot at parties

coherentpony5• 13 years ago

Oh, you haven't the slightest.

aolol• 13 years ago

Perhaps you could elaborate on this. I do agree that the title and video give slightly more credit than is due based on the current state of fluid dynamics knowledge. However do you mean your group is searching for closed-form solutions to NS or are you generating approximate numerical solutions (CFD)?

coherentpony5• 13 years ago

Approximate solutions are found computationally.

Non-trivial closed-form solutions are hopeless. Existence of solutions is a little less hopeless. Existence of solutions over small time horizons has been proven. This is all in three space dimensions.

There is a theorem stating that if global-time solutions exist and are unique, then computationally computed approximate solutions are 'good'. Moreover, they get better as you refine the computational domain.

starpilot• 13 years ago

At this time NS solutions are all approximate, but there's a huge gulf between plausible-looking fluids (as in story), and fluids with pressure, velocity distributions accurate enough for analysis (to design an airplane). And within the latter there are further demarcations depending on the type of fluid analysis, where errors manifest as extremely subtle (pressure looks slightly high, vortices dissipate too quickly or too slowly) and judging the best algorithm for a problem becomes almost an art.

mercnet• 13 years ago

I am SQA Engineer for Autodesk Simulation CFD and our recent 2014 version has free surface modeling: https://www.youtube.com/watch?v=WNKHf937Qt0. It does make pretty pictures but as you mentioned solving navier-stokes equations can get complicated :)

coherentpony5• 13 years ago

Beautiful. Those boundary conditions must suck!

Vivtek2• 13 years ago

The only uncanny valley effect here is that the water doesn't wet anything - everything is perfect Teflon. So it's hard to judge whether the movement of the water is perfectly realistic, because I keep looking at the transitions. If I don't look at surfaces, it seems very convincing.

mtgx• 13 years ago

The color is also not right. But the animation seems almost flawless otherwise.

pionar• 13 years ago

Really? The color's not right? Not all liquids are the same color. Hell, not all water is the same color. But the GP is right when the water doesn't wet anything, it's kinda eerie.

EDIT: Having said that, water simulation is HARD.

Vivtek2• 13 years ago

Oh, you people with fully functional color vision are always complaining about this kind of thing. I'll just stand over here with Van Gogh.

hmottestad• 13 years ago

Every once in a while you can see a "pice of water" that should be smaller but is limited by the size of the underlying spheres.

mixedbit• 13 years ago

Also I think a cube of water at 1:50 that holds its shape until it hits a rabbit does not reflect how a real water would behave. But overall I find this simulation very compelling.

aethertap• 13 years ago

This should lead to much more interesting interactions with water in games, because now it has actual physical properties instead of just being something pretty to look at. Watching the water flow out of the aquarium makes me think that water is about to become a real active part of the game environment. Very cool.

FuzzyDunlop• 13 years ago

I remember Super Mario Sunshine was much lauded in the games press because of what it did with water and the effects. I absolutely loved the game but you don't really hear much about it any more.

kvprashant• 13 years ago

Somehow, I felt wow. I am able to appreciate this because of the number of applications it holds - Tsunami simulation, Bridge and dam construction. And real-time? Full points!

jjoonathan• 13 years ago

Artistic water simulation and scientific/engineering fluid simulation tend to be mutually exclusive in that they optimize for different objectives. Typically, a solution in one domain is ill-suited for the other. For instance, the "Stable Fluids" trick leads to desirable artistic properties (fluid behavior more closely matches stage direction = active energy input) at the expense of systematic inaccuracy (total energy is biased downward) that would be undesirable in an engineering context.

If this technique worked equally well in both domains it would go from "impressive" to "miracle". In the meantime, we can certainly applaud it for the things it does do well!

SiVal• 13 years ago

Does anyone here know what technology stack this is implemented in/with/on?

angersock• 13 years ago

C/C++/nVidia (formerly Ageia, formerly Novodex) PhysX.

pcl• 13 years ago

Cool stuff, but the viscosity seemed a bit off (especially noticeable when the glass walls were shot out towards the end), and waves seemed to hold their shape for a bit too long. I wonder how hard that'll be to tweak.

timdorr• 13 years ago

I think the nVidia real-time ray tracing demo is more impressive: http://www.youtube.com/watch?v=w9SH8xlgzoI&feature=playe...

Primarily because of the distortion of the visuals through the water. This simulation fakes it somewhat, which you can see with the distortion being somewhat blocky.

spoiledOldGuy• 13 years ago

That is so god-like! I give up I do not want to program anymore.

xiaoronglv• 13 years ago

very cool

Keyframe• 13 years ago

Impressive for real-time, but nowhere near realism one can get with Naiad or Realflow and a proper renderer. For example: https://vimeo.com/37536300

missing_cipher• 13 years ago

I prefer their ocean simulator: http://www.youtube.com/watch?feature=player_detailpage&v...

nobodysfool2• 13 years ago

I've never seen water simulations that can pass the U test. Basically, take a pipe, bend it into a U shape, pour the water in one side, and see if the water levels even out. If it does, then it's a very accurate simulation of water.

andypants• 13 years ago

What happens instead in the simulations that fail?

samatman• 13 years ago

they seesaw for far longer than is reasonable.

nobodysfool2• 13 years ago

usually the water fills up the bottom part of the U until it reaches the width of the pipe, the only one side fills up (the supply side). So if I fill up the left side of the U pipe, the water forms an L shape. On some simulations, the other side rises, but doesn't really even out.

magoghm• 13 years ago

I agree, the U test is very hard. I spent many days trying to make a working 2D approximation with cellular automata, but I couldn't find any way to make the pressure propagation fast enough. The water would eventually level out on both sides of the U, but it took a very long time to do it.

panic• 13 years ago

I think that's backwards -- if it's a very accurate simulation of water, it will pass the U test, but you can add hacks to make a simple, inaccurate system pass it, too. See, for example, Dwarf Fortress's pressure system: http://dwarffortresswiki.org/index.php/DF2012:Flow#Fluids_un....

tome2• 13 years ago

This reminds me of a much older open source game where two players controlled liquids and the aim was to engulf the other liquid. I can't remember the name of the game, though.

cbhl• 13 years ago

liquidwar?

tome2• 13 years ago

Yes! What a funny game.

rip747• 13 years ago

honestly, after watching the demonstration video, there is nothing in the video that i haven't seen in numerous hollywood films or games before. am i missing something? would someone care to explain what i should be looking for?

gknoy• 13 years ago

I've never seen water simulation with this degree of complexity in games. I've seen it in prerendered video sequences, but when you sit and look at a lake or river in an Elder Scrolls game (or similar), it is usually much more simplified -- or has a flow which is modeled to look pretty.

The difference here is that it's showing interaction between the water particles and things that they impact on: characters, boundaries, piers, and behave in a more realistic way than games have previously had.

alexvr• 13 years ago

Yeah, that does it. We live in a computer simulation.

Introduction to Position Based Fluids

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