Hair simulation 101
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Hair simulation overview
Hi, I'm Hayley Iben I'm a software engineer at Pixar. I worked on the hair simulator for the movie, Brave. Hair was a really big deal in Brave. It was a symbol of freedom that was intrinsic to our main character Merida's personality. And making hair that feels right, meaning messy, wild and free, is a hard thing to do computationally. In our movies, hair is a simulated effect. That means no one animates its movement by hand. The way it moves is defined by physics and programmed into the computer. To figure out what hair simulation should look like, we needed to look at real curly hair and see how it worked. What we found was this. Curly hair forms together in locks that reshape themselves when you pull on them, kind of like a spring. This was great news because springs can be modeled mathematically, meaning we can simulate their behavior using a computer program. From the computer's perspective, Merida's hair is just lots and lots of springs that react to forces, such as gravity, as Merida moves. The tests we ran using this approach were pretty good, but not quite natural enough. While stiffer springs held the shape of the curl, it didn't bounce like natural curls. When we made the springs loser, her curls would unwind when she moved. This was even more obvious at high, often unrealistic speeds, found in animation. We realized we needed something to preserve the structure of the curl, but not impede the overall motion of the hair, kinda like digital hairspray. (can hissing) Oh, sorry (laughs). We came up with the idea of connecting the springs that make up Merida's on-screen hair to invisible core springs like this. The core springs would limit the movement of the on-screen hair when the motion was more extreme. This way, we could have the best of both worlds. Our original simulation we create motion that was soft and natural, but the core springs would keep the curl from unwinding too much. Figuring out how to make a hair simulation system that could help us achieve the feeling we wanted for Merida's hair was hard. It took a lot of iterations to get it right, but it made a big difference. Merida feels like a real girl, messy, wild and free.