A CGS article about all the new techniques Pixar use in Brave, like Merida's hair.
from the CGS article:
Pixar Animation Studios started toying with their hair simulation about 12 years ago, within the throes of creating Monsters Inc. Then in 2009, with the beginning of BRAVE the 13th animated production at the studios, a research project was begun to work out how to depict curly hair. The following year, this was brought into production for Merida’s hair in BRAVE.
“Hair is often very shading and lighting dependent,” adds Claudia Chung, Tailoring Cloth Supervisor at Pixar. “In terms of simulation, the core simulator we used on BRAVE for cloth is exactly the same one we used all the way back to Monsters Inc., even Geri’s Game.” Of course over the years, through Ratatouille and The Incredibles, a modeling suite was written for the simulator to create the costumes for Toy Story 3 and the same system was used to create the assets on BRAVE. The idea of hair-to-hair interaction was important for BRAVE, because the bounce and free flow of the hair was particularly important for the Merida character to be accepted. She had to have hairs out of place. So, in terms of hair, it was a completely new simulator.
Figuring out how the hair would move was also a huge challenge for Claudia and the team. “The main character Merida had some ideas of how life was, which are challenged during the movie,” Chung says. “Life is more complicated than she thought.” Claudia Chung found that 3D CG hair had a life of it’s own as well and sometimes things didn’t go that way she thought they should either. It was a great adventure for both Merida and Claudia.