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After watching Brave, one thing that I was curious about was the curly hair of the main character, Merida. It’s so long! It bounces and everything!

How is it possible to animate that hair for every frame?

picture of Merida with a bow

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    Its not just the hair, either...the soft matter physics in general was superb. Beautiful cloth in motion sequences for a multitude of different weights and textures. Wet cloth that looked right in general and in relation to the same material when it was dry. Absolutely fabulous.
    – dmckee --- ex-moderator kitten
    Jun 23, 2012 at 21:00
  • I remember watching a Pixar promo/interview (can't remember the name...sry) circa 2001 where someone in the Pixar crew said they had no program to render hair for Sully; so they wrote one. In the IMDB triva of monsters, inc, they mention that it took 11-12 hours to render a single frame for Sully because of all that hair. I'd be interested to see if they reused some of that code and if they waited just as long for rendering in Brave.
    – ray
    Jun 24, 2012 at 2:09
  • I recall hearing something last week about how they had to overhaul their hair-simulation software for Brave because Merida’s hair is curly, unlike the hair of almost all of the other animated characters so far. Unfortunately I do not get Computer Graphics World anymore, but I’m certain that CGW will do an article on Brave (specifically about the hair simulation) in a month or two.
    – Synetech
    Jun 24, 2012 at 18:23
  • The answer to this is probably very 3D software-indepth. My guess though, is that they used some sort of algorithm that makes the hairs move with an (almost) identical movement.
    – paddotk
    Jun 25, 2012 at 14:43

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There are automatic hair generation and physics-based animation bolt-ons available for most common commercial CGI creation/rendering packages, these have been around for a long while really.

That said Pixar do tend to write their own software and will probably have done so for 'Brave'.

Essentially you define the area on the head where hair is to be, you can then define length, colour, drop-off areas etc. and the hair itself is then procedurally created. It's then down to the physics package to handle how it moves when moving/turning etc. and the rendered is told the reflection/refraction expected of the hair so just gets on with the job.

It's a lot of complex calculations but it's far from 'rocket science' these days, realistic shadows and flickering light are much more challenging.

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  • One thing though, Pixar has been taken over by Disney many years ago :) So it's disney then
    – paddotk
    Jun 25, 2012 at 14:44
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    A small semantical addition: when the movement is rendered through physics calculations, that's generally called "simulation" whereas "animation" is movement imparted by the designer themselves (eyeballed). Merida's hair would be too much to manually animate even if the animator was highly skilled. The semantically pedantic answer to the question is "it wasn't animated, it was simulated".
    – Flater
    Nov 8, 2018 at 7:17
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Khan Academy partnered with Pixar a while back to provide an introductory course to computer animation. One of the lessons gives a simplified explanation of how they animated Merida's hair. You even learn to write some simple code to accomplish the basic idea!

Hair simulation 101
Explore how millions of hairs can be simulated using a mass spring system. This lesson is appropriate for grades 5+ and takes approximately 1 hour to complete.

Here's the introductory video:

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