The Science Behind Hollywood Explosions

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.

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