Untangling The Hairy Physics Of Rapunzel

Kelly Ward, senior software engineer for Walt Disney Animation Studios, was tasked with bringing Rapunzel's locks to life in Disney's Tangled. The hair had to look realistic, but not too real — otherwise Rapunzel would be towing 80 pounds of hair behind her.

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IRA FLATOW, HOST:

Joining us now is our multimedia editor Flora Lichtman who is - welcome to SCIENCE FRIDAY, of course.

FLORA LICHTMAN, BYLINE: Thanks.

FLATOW: We'll have a backend. We have our Video Pick of the Week today, sort of an oldie goldie, right?

LICHTMAN: Yeah. But it's related. So we thought we would bring it up. It's a hairy quest scientific question. I'm just going to fill it with bad jokes. It's about the physics of Rapunzel's hair, is the topic of this week's video pick. And we talked with Kelly Ward, who's a senior software designer for Disney, and she had studied hair modeling in school. And I didn't even know this was a field you could study, but it just sort of gets to what we were talking about - become really good at something. And then she found out that Disney was making this movie about Rapunzel, and it was like the dream fairytale ending for a student of hair modeling.

FLATOW: So it was the film "Tangled" and she - and in your video pick, she talks to you about how she had to mathematically model the hair and different...

LICHTMAN: It's a - yeah, it's a really complicated question, it turns out. So we have over 100,000 strands of hair on our real heads. In this movie, they look at - they sort of boil it down to over 100, but even that is a really complicated question. I think Tony DeRose could probably talk about this better than I could. But, you know - is the problem that they bump into each other and the static? I mean, how do you think about this problem of hair movement?

DR. TONY DEROSE: As you said, there are about 100,000 hairs on a human head. There are many more on an otter.

(SOUNDBITE OF LAUGHTER)

FLATOW: Is that why we don't see many otter movies?

DEROSE: That's why we don't see many otter movies - yeah. But if you can capture the essential motion of about 100 hairs or so or 200, then you can kind of do in-betweening for the hair and get the basic motion at dramatically lower simulation cost.

LICHTMAN: And, you know, I mean, that's the one thing that Kelly mentioned, that it takes a long time to render this kind of stuff, right? Can you give us a sense of how long it takes to actually output a shake of the head?

DEROSE: Well, the outputting isn't so time-consuming, but doing the light simulation of how light interacts with all the hair and how the hair shadows each other, that's extremely computationally intensive.

FLATOW: Give us a how long. For a second or two of - Steve, how long would it take?

DR. STEVE SULLIVAN: It could be overnight for a single frame.

FLATOW: Overnight for one frame.

SULLIVAN: Yeah. Depending on the creature and what's going on. Another interesting aspect of this is why we have to get the physics right so it's kind of believable. We have to then put controls on top so animators can make the hair perform, because what you see in the Rapunzel footage is that hair is doing something that real hair couldn't do. It needs to express something about the story there. So a lot of the challenge for our technologists is to come up with interesting controls for the animators to create performances out of physically based things.

LICHTMAN: Yes. This is exactly what Kelly Ward said. If you actually had Rapunzel's hair, you'd be dragging around 80 pounds.

SULLIVAN: Right.

FLATOW: Right.

LICHTMAN: You wouldn't get that.

FLATOW: I'm Ira Flatow, and this is SCIENCE FRIDAY from NPR.

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