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MillimeterDeveloping Drop
Jan 12, 2010 12:00 PM, By Ellen Wolff
How Digital Domain arrived at a new simulation system.
Drop test: multistory shaded model collapse
The realities of crashing buildings for 2012 put the new tool—aptly named Drop—to the test. Stephens explains the process. "We'd start off with something in a Maya scene, where an animator might indicate where a building should start to tip," he says. "That would be the first part of the puzzle. We'd have all the constraints binding different parts of a model together. The model would be effectively 'slaved' to that Maya animation just to get it going. At some point, the simulation would be switched to live, but only in pieces—so that parts of the building would be simulated and others were still under a more kinematic influence. Eventually we'd also animate the constraint strengths between pieces so things could crumble more on cue. By an artful use of those tools, we were eventually able to art-direct, more or less, what a simulation would look like. You'll always have a certain amount of randomness—that's just the luck of the draw—but we've basically stacked the deck as best we can to get the timings and the basic blockings right. Then we can iterate to finally find the exact-right simulation.
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"Directable chaos is the name of the game. You know you want something to fall a certain way with a certain timing. But if the director wants it 4ft. closer, that's a directable goal. At that point, your goal is to get the right physics look to it—but the fact that you can influence it to hit a mark is the controllability part."
Control was one goal for the team on 2012; speed was another. "Speed is the principal thing we have to accelerate in all simulations," says Clive, who previously worked on DD's simulations for Pirates 3 and The Mummy: Tomb of the Dragon Emperor. "With speed, you get artistic control and quality. You can have the world's best sim, but if it takes 40 hours to run, you won't be able change anything. You'll get stuck in a loop with a 40-hour waiting period. With Drop and the technologies associated with it, you get the ability to have direct control over what's occurring in the simulation, which is really valuable from an artist's perspective. If you don't feel you have that control, you won't be able to address the notes you get from the director. You won't be able to make that corner of a building fall four feet closer because it will take 20 hours to generate that. With Drop, it's 45 minutes."
"One of the nice things was that we were able to do sims in layers," Stephens says. "For example, we could have a couple of hero buildings destroyed and then use those sims as collisions for the next layer of collapsing buildings. If you tried to do that with miniatures, you'd have to have 15 to 20 buildings all collapse on cue somehow to get a directable performance. That's obviously not going to happen. But with this approach, you can work from the inside out. You work on the ones you know you want to make the hero buildings of the shot and then have all the other sims work off those. And if the director thinks you blew up too many windows, you go back and re-sim it, leaving more windows intact. You don't have to re-run the original building sim; you just have to re-run the secondary sim. The ability to layer these sims gives you enormously powerful artistic control."
Drop test: shaded model house collapse
"Another aspect of Drop that is pretty neat is that we're taking for granted that stuff is fractured," Zafar says. "It doesn't come out of modeling that way. There are a bunch of tools that perform the artisticially driven fractures into the correct shape. Then it goes into a phase that rigs them up in custom ways and the rigging reflects the kind of materials they're made of, whether it's rebar or concrete. The behavior is different, and the rigging tools take care of that. Perhaps that's where most of the novelty is—how the fracturing and rigging is done. If you look at a Drop setup, there's really just one simulation node. There are a bunch of other Drop nodes that do all the other stuff."
"The entire architecture was designed to save the most amount of work to be done in the last step," Clive says. "Everything that you want to do that's quick and easy you can do as prep work. If you were a cook, it would be like dicing onions and putting them in a bowl until it's time to use them. You do your prep work in a fast, lightweight way, and then you throw them into the heavy sim. So it doesn't slow you down. This wasn't just used for buildings—we had an open space sim where we ended up making car rigs with turning wheels and bending axles. We even figured out how to make trees bend and shimmy during an earthquake."
The end result was an extensive library of simulations available to the production team of 2012. "We had some airport buildings that had very large structures that we built libraries out of—we would run these sims out for 1,000 frames or more," Stephens says. "A couple of those would cook overnight, so we then had thousands of frames of extremely detailed buildings in all stages of collapse. We could then slug them in anywhere."
Digital Domain's development team says that the Drop system created for 2012 reflects an industry trend. "We're getting the technology to a point where we're freeing artists from having to be technicians," says Stephens, who is currently working on Tron Legacy. "At the moment, Drop is still very technical to use. But over time, I think we'll end up seeing more intuitive interfaces. And as machines get faster and algorithms get better, the time of iterations will go down as well. We'll continue to see this approach bloom. There's no going back."
See Digital Domain's Reel-Exchange profile and reel
Continue the discussion on “Crosstalk” the Millimeter Forum.