For Brave, the team at Pixar had to deal with a hero or rather heroine, who is on screen for almost every shot, but who needed wild, yet beautiful hair. The simulation department needed to develop a technique and approach not only viable on a hand crafted trailer shot, but something that could be used almost ‘out of box’ on most shots, or the film was simply never going to make its deadline.
Warning: this article contains plot spoilers.
The Princess Merida’s hair was almost a three year process to get correct. Earlier hair such as for the characters in The Incredibles had much more groomed hair, but here the character’s hair simulation needed to solve how to get this messy, tempestuous and unpredictable look while still looking attractive and flowing nicely with the animation.
The previous hair system used by Pixar had to be seriously overhauled since:
Merida’s hair required much greater hair to hair (soft body) collisions than anything Pixar had done before.
The curls themselves were an issue from a simulation point of view.
Hair is modeled using a series of mass and springs (springs connecting point masses). But very curly hair acts in a contradictory way: curls are very stiff, so the springs need to be quite rigid, but the movement of the hair requires a soft flowing quality that fights this property. If the ‘springs’ were too soft the hair would unwind, if the hair springs were too stiff, her hair would not move realistically. This coupled with the problem of collision and intersection – while holding to the production schedule – made Brave’s hair simulation extremely complex.