
A Material Point Method For Snow Simulation - fekberg
http://www.disneyanimation.com/technology/publications
======
bhouston
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](https://en.wikipedia.org/wiki/Material_Point_Method)

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batty
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.

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bhouston
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.

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tinco
0.5 * 10^-3s is 2000hz, so 20 iterations per frame, assuming they render at
100hz, that seems a bit on the high side.. does that mean they don't have a
good way of interpolating the physics effects, does that happen a lot in these
kinds of simulations?

I work on a real-time multi-player game, and our physics step is actually
larger than our framerate, we interpolate the rest, I guess that's quite the
opposite of this :)

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bhouston
Film is rendered usually at 24fps or if you see the Hobbit it is HFR at 48
fps.

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nitrogen
In the case of the simulation, the question is whether running the simulation
at 24Hz or 48Hz would result in inaccuracies that require them to run it at
2000Hz.

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chaz
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](http://www.youtube.com/watch?v=JAFhkdGtHck)
(3 min)

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moconnor
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
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.

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

[1] [https://disney-
animation.s3.amazonaws.com/uploads/production...](https://disney-
animation.s3.amazonaws.com/uploads/production/publication_asset/72/asset/snow.pdf)

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xerophtye
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

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berkut
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.

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

~~~
DannoHung
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?

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legacy2013
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.

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javindo
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!

~~~
NamTaf
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.

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nitrogen
Perhaps the link should be
[http://disneyanimation.com/technology/publications/55](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).

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ByronFortescue
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.

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guardian5x
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.

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chii
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)?

~~~
visakanv
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](http://www.youtube.com/watch?v=BhzMR-vxYIk) (Skip to about 2:50)

~~~
nitrogen
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?

~~~
visakanv
I suppose it makes you feel more involved, rather than just watching a bunch
of cool cutscenes that you have nothing to do with (common in stylish Japanese
RPGs like Devil May Cry, etc)

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rschmitty
This got no love before
[https://news.ycombinator.com/item?id=6780984](https://news.ycombinator.com/item?id=6780984)
oddly

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

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frogpelt
There's probably a time limit on that function.

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Cort3z
Coming from a snow-filled country I would say that this simulated snow behaves
very close to the real deal. Very impressive

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clord
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!

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Kerr0r
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?

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trekky1700
Is it weird that I find this mesmerizing despite there being a foot of the
real stuff outside my door?

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

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

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gyom
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).

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DoggettCK
Beautiful, and makes me feel like a complete fraud.

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dakrisht
Amazing

