A Material Point Method For Snow Simulation

http://www.disneyanimation.com/technology/publications

by fekberg • 13 years ago

310 60 13 years ago

bhouston3• 13 years ago

This is such a beautiful result.

The simulation method used is Material Point Method, a technical that is used in engineering simulation but hasn't really been used much in Visual Effects. This will likely change now that there are these impressive results.

The one issue I've seen people in the industry mention about this paper is that the the simulation times are quite long.

https://en.wikipedia.org/wiki/Material_Point_Method

batty• 13 years ago

Agreed, very nice. I've heard the speed issue mentioned as well, although slow compared to what, I wonder? I don't see offhand any fundamental reason why it would be dramatically slower than, say, a standard FLIP-based fluid sim. It amounts to advecting Lagrangian particles combined with solving the elastoplasticity equations on a Cartesian grid, with a snow-specific constitutive model.

bhouston3• 13 years ago

There min/frame in the included paper chart range from 3.8m/frame to 25.8m/frame, which is pretty slow for 2013 at the simulation sizes they give.

It is too bad they didn't break out the individual steps to show where the time was spent. Could simply be unoptimized code. I notice there is a restriction on the time step to be less than 0.5 x 10^-3 s, not sure if that is the main difference.

chii• 13 years ago

I dont know anything about rendering or simulations, but would this method be possible for real time rendering? or is it only applicable for offline rendered stuff (like movie sfx)?

visakanv2• 13 years ago

I know very little about it too, but I think advances in video game visuals have shown that it's really just a matter of time. Check out Uncharted 3's real-time dynamic water effects: http://www.youtube.com/watch?v=BhzMR-vxYIk (Skip to about 2:50)

PavlovsCat• 13 years ago

Skip to about 2:50

I think you meant to say 5:20 ^^

tzakrajs• 13 years ago

Battlefield 4 does a better job with real swells in the ocean allowing you to fight with boats using waves for cover.

nitrogen3• 13 years ago

I noticed some moments in that video where a flashing controller button appeared in the corner. Does that mean the player has to mash the button to continue? I hate games that do that. Why do game makers do that?

babby• 13 years ago

I don't know how complex the computations are, but if it's close to the computing required by standard loose-accuracy water fluid simulation, we could certainly see it in-game, definitely possible even for this gen of consoles. I speculate as water simulations is quite doable even on 5 year old hardware, though tends to be inaccurate. I remember seeing some Nvidia demos showcases things like softbody destruction and collisions a few years back too, so that also could translate.

There are also some really rough tessellation techniques in use to simulate ground snow walking in games out now, like that Assassins Creed with the native americans.

berkut3• 13 years ago

Proper water sims for large scale VFX production using things like Naiad, Houdini or RealFlow can take more than 12 hours per frame:

http://lesterbanks.com/2012/03/10-great-examples-of-naiad-in...

It depends on the accuracy of them and the resolution / speed they're going to get rendered at.

russellsprouts• 13 years ago

This simulation takes on the order of minutes per frame. Perhaps an optimized and simplified version could work in real time in the future.

poizan42• 13 years ago

If we assume 5 minutes per frame then even without optimizations we should be able to render 25 frames per second in around 15 years (assuming Moore's law continues to hold)

moconnor• 13 years ago

The video is well worth watching. After several minutes of dry technical commentary, you suddenly get "Now we destroy a castle with a cannonball!"

The snowballs colliding is extremely impressive.

bilalq• 13 years ago

The guy talks way too fast though. I'm really struggling to understand what words he's saying. It almost feels like it's artificially accelerated.

That being said, the visuals do indeed have a "wow" factor to them.

mischanix• 13 years ago

The pdf linked [1] covers in-depth what the narrator is saying.

[1] https://disney-animation.s3.amazonaws.com/uploads/production...

berkut3• 13 years ago

It's for Siggraph, so there's a time limit.

pvarangot• 13 years ago

If you know what is Cauchy's stress tensor, know about finite element analysis, and grad classical mechanics, its understandable. If you don't know about that, you will not learn it in a demo...

dakrisht• 13 years ago

Amazing

chaz• 13 years ago

I don't know anything about graphics and simulation, but I'm always impressed with these SIGGRAPH demos. Here's the demo trailer for the papers from this year: http://www.youtube.com/watch?v=JAFhkdGtHck (3 min)

ByronFortescue• 13 years ago

It looks pretty good, but still, I think some of these examples look really foamy, and not like real snow. Not all physical behaviour is yet replicated I think.

guardian5x• 13 years ago

Of course not, its only a simulation of a model. The data necessary to store all physical attributes including every atom and sub-atomar particles would not fit on any hard disk in the world. The result is nonetheless pretty amazing.

Dirlewanger• 13 years ago

We're getting there. Looks more like wet sand though.

pistle• 13 years ago

You must have grown up with snow. My thoughts exactly. It fell into the uncanny valley for me... Sort of close, but then off in unnerving ways.

Great work for where they got to, but it's not quite right yet.

ihsw• 13 years ago

As someone that grew up with snow and been to a beach more than I'd care to admit, I can't help but agree that it's quite the uncanny valley.

I'm sure Disney et al will find ways to distract you enough not to notice.

I'm far more interested in seeing snow falling, especially with (for example) a street-lamp contrasting it. That would be something to marvel at.

DannoHung• 13 years ago

It looks mostly right for wet snow. Powdery snow wouldn't work like this though. Maybe they just didn't think that example was very interesting?

sparky• 13 years ago

Yeah, the paper talks about how to vary the parameters to simulate powder, wet snow, slush, etc. These techniques were developed for the movie Frozen, and to my eye (I grew up with snow too) they did a great job of simulating all manner of snow and ice.

legacy2013• 13 years ago

Yea, powdery snow isn't all that hard to simulate (relatively) I believe. The snowflakes are just particles that are acted upon by an outside force( wind usually). I live and grew up in New England, and the snow in Frozen looks completely realistic.

It's amazing the amount of work put into these movies. Most moviegoers won't realize what it took to simulate snow realistically, but the would have noticed if it was wrong.

orbitur• 13 years ago

In the video, the clumpier it is, the better. I would say the more powdery snow looked like wet oatmeal when it hit the floor.

zemo• 13 years ago

...have you seen Frozen? Go see Frozen.

micro_cam• 13 years ago

I'd like to see some simulations of slab avalanches...The stuff with wet sticky snow looked okay but I think they missed the ability of even low density snow to act cohesively as slabs [1] (which leads to some of the most deadly avalanches). Need to review their math but maybe they only account for particle adhesion and not more crystalline structure (which would obviously make the simulation more expensive).

The woman walking (post holing is the technical term) looked off for the same reason...the snow didn't look slaby/crusty enough to act like it was.

[1]http://2.bp.blogspot.com/-4pGis30qlj0/TwcAGuSz8fI/AAAAAAAAAW...

kolev• 13 years ago

Yes, wet, too sticky. Still pretty good, but you can clearly tell that it doesn't behave like real snow just yet.

javindo• 13 years ago

I was talking to my father in law (head of particle research for P&G) about particle simulation for research purposes and he was explaining the complex challenges with behaviour of interactions. It amazes me that even at the mathematical level, a post-doctorate doctor of chemical engineering with a team of physicists/computer scientists/mathematicians/chemists are still facing challenges which Disney seem to have solved - glad to see they are properly releasing this research!

fsloth• 13 years ago

The requirements for a visual simulation and industrial process simulations are quite different.

NamTaf• 13 years ago

This process yields fantastic, realistic visuals, but I bet you'd find it does not yield the scientific information they're chasing. It's sort of like how the 'standard' linear static finite element method assumes insignificant displacements and no plastic yielding of materials - as soon as you need to deal with any sort of significant level of displacement or distorsion of the material in question, a linear static solver isn't capable of giving you accurate results.

With displacement, it tries to fake it by extrapolating the linear displacement and assumes that applies over the entirety of it, when in actual fact as soon as it displaces, the strain field in the material will change and it needs re-solving.

It's a good tool, but it has its limitations.

rschmitty• 13 years ago

This got no love before https://news.ycombinator.com/item?id=6780984 oddly

GFischer• 13 years ago

This title is certainly more catchy. It's odd it didn't upvote the prior link.

frogpelt• 13 years ago

There's probably a time limit on that function.

GilbertErik• 13 years ago

Goes to show how much a catchy title matters. :-/

scott_s• 13 years ago

The front page of HN is like many other venues for what becomes popular: there is an enormous amount of randomness.

joelthelion• 13 years ago

It's neat, but their snow looks more like old heavy snow than recent powder.

atrus• 13 years ago

Even better, that's what they were trying to simulate.

gyom• 13 years ago

Except that, if it was what they wanted, then it's hard to explain the walking character at 3:30 that gets his legs almost trapped in the soft heavy melting snow.

There's just no way that you'd get that kind of effect with that kind of snow. It looks more like the kind of thing that you'd get from non-uniform snow that has a crust on top (and that kind of snow is not sticky and not melting).

xerophtye• 13 years ago

What i really like about this is that instead of keeping it under wraps and calling it a "trade secret", they openly discussing their technique and releasing it for the advancement of the field. Not only did they write a paper about it, but made it open access! (as opposed to traditional journal publication)[1] So kudos to their effort!

[1] Though this IS disney we're talking about and they certainly don't need journals for getting an audience for their papers

berkut3• 13 years ago

Most of the VFX industry releases the majority of their techniques as papers, etc. Some of it's kept a secret, but that's happening less and less.

There's a lot of competition, but still there's a lot of open stuff like OSL, OIIO, OCIO, OpenSubDiv that multiple facilities contribute to.

Cort3z• 13 years ago

Coming from a snow-filled country I would say that this simulated snow behaves very close to the real deal. Very impressive

clord• 13 years ago

Yep, but I'd qualify that: The demo seems to show wet snow (snowball snow) only. There are a few other types that happen around here, like dry snow, and crystallized snow. Dry snow is light fluffy stuff that doesn't pack. And crystallized snow is made of what feels like little daggers, behaving almost like ball bearings. But still, their result is brilliant!

officemonkey• 13 years ago

Have you seen "Frozen?" It really was a beautiful-looking film with lots of opportunities to show off the simulated snow they modeled.

DoggettCK• 13 years ago

Beautiful, and makes me feel like a complete fraud.

trekky1700• 13 years ago

Is it weird that I find this mesmerizing despite there being a foot of the real stuff outside my door?

nitrogen3• 13 years ago

Perhaps the link should be http://disneyanimation.com/technology/publications/55 (note the added /55) so it goes directly to the post in question, even if the page is updated (permalink was obtained by clicking the Twitter icon).

Kerr0r• 13 years ago

As a CS major this fascinates me to no end, but reading the paper the physics are a bit much for my brain to handle. Can any physics-inclined people here recommend me some good books/papers so I can learn more and hopefully make more sense of this?

A Material Point Method For Snow Simulation

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