What we focus on is the simulation of complex natural phenomenon. We are mainly
concerned with it from an aesthetic perspective — so essentially what looks
interesting. We use a lots and lots of the same tools that they use in physics or
computer science, but we of course adapt them to our own special needs. Around the early
2000s, computer-generated smoke and fire, they really started showing up
and looking very nice in films and this was because an algorithm was developed
that actually deals with very broad, fluid swirls. This is actually the icon that
you would draw for steam rising off of a mug or even when children draw wind.
So obviously the swirls are what is the visually important aspect. The problem is
when you want to do something larger like a house on fire. In that case, the
visual aspect that is missing is that there are smaller swirls that are
layered on top of the large swirl and these are really what sell the idea that
something much larger is going on. These algorithms, they are capable of doing
this sort of thing, it's just it takes a very long time to compute them so
instead of waiting maybe a few seconds to see if the mouse click had the
effect you wanted, you might have to wait a few hours or maybe overnight. We
published this paper called Wavelet Turbulence back in 2008 that dealt with smoke
simulation and at the highest level what wavelet turbulence did was it made it
much faster to get the details needed in order to make something look like a
forest fire. As with all science, we have to build on research from other universities. So
there was a very nice algorithm and they actually showed how to make these little
swirls very, very quickly. So we looked at that and said, "maybe there's a way to
actually couple this to the larger scale flow." This actually was not a new idea and
the problem was if you have these tiny little swirls everywhere, it does look a
little bit larger, but
the ones that are, you know, way out here it does look wrong. So we had to answer
the question: where she's tiny swirls actually appear and where should they
not appear? And for this we used a tool called a wavelet transform. Once you've injected
these tiny vortices, they do need to move along with the flow. So we added another
detail to pull these along with the flow.
This is a method called texture advection. There was one last piece
though
that was creating some visual problems for us. Say we have this vortex. The flow
that's pulling it along, it actually pulls things apart. There's actually the
opposite process as well. You have a large flow that's actually pushing them
together. And we were seeing that this physics was missing. So what we did was we
built a Jacobian matrix. This will actually tell you which situation you
may be in. We checked if we were in either of these situations and if it was we
actually removed that whirl. It seemed to fix the visual artifacts that we'd seen
before and with that we considered the algorithm complete. When we presented a
paper at the end we said, "oh and by the way you can play with the source code if
you want to." So we sort of released into the wild, then a colleague that we knew a
DreamWorks actually contacted us saying well this looks kind of interesting. They
were working on Monsters vs Aliens and our tool had actually made it in to production fast
enough that they used it for the final sequence. So that was the first time we
got to see it on the screen. It was used during the train crash sequence in Super
8. It was really exciting to find out about that because when I first saw the movie
I didn't actually know it was used in that and it was spectacular sequence in its own
right. But maybe my favorite movie going experience was my colleagues had told me
beforehand that it actually was used in Iron Man 3 so when me and my wife went
to go see it we actually both could point at the screen at the exact moment
that it appeared, so we had that nice little private moment in that theater.
And that was a nice experience. Ever wonder how filmmakers created special
effects before computers? Watch how it was done during the silent era in this
video here and make sure to subscribe to figure 1.