Hair

It was pretty much suicidal choice (Or would take A LOT of patience and time) if one decided to attempt drawing fluid and realistic (And I mean where every single strand of hair could actually be seen) hair frame by frame when it came to animating something in 2D, and so it was mainly the volume and bounce that was focused upon to show off the character's gorgeous hair. But of course, 3D animators always had to take it one step further, especially with all the tools softwares such as Maya provided them.

For "Brave", 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 be not just messy, tempestuous and unpredictable, but also attractive and flowing nicely with the animation.

It is because of that that the previous hair system used by Pixar had to be seriously overhauled since. Hair is modelled using a series of mass and springs. But very curly hair acts in a contradictory way (Curls are very stiff, but the movement of the hair requires a soft flowing quality that fights this property. And if the springs were too soft, the hair would unwind and if it was too stiff, the hair would not move realistically), this along with other problems such as collision and intersection made the hair simulation for Merida extremely complex.

Pixar ended up developing and internally releasing a new simulator, which is multi-threaded. The new code does computation before it sends hair off to the simulator to determine how other hairs would relate. This means that a cluster of hairs would all be dealt with as one group, and the hair simulation could be multi-threaded.

In one sequence, Merida flicks her hair from one side of her face to the other, but even such high level gross spatial charges were able to be automatically accommodated.  

More can be read here on how much went into not just Merida's hair but the hair of the other characters as well as the fur for the animals.

Rapunzel's (Was it a coincidence that Disney decided to make this their first 3D animated Disney Princess film?) excessive hair length in addition to the loose style and intricate structure of the hair presented many unique challenges in hair simulation. Moreover, the specific art direction of the film adds to the complexity of the simulation.

To simulate her hair, a software known as dynamicWires was used, which utilised a mass-spring system for the dynamics of curves. Its system was extended to handle the challenges of Rapunzel's hair. Such features including hair piling, volume preservation, effortlessly dragging (To show just how silky smooth her tresses are), simulation freezing (That freezes the back part of the hair that allows them to adjust the length of simulated hair per-shot, accelerating the run-time of the simulation significantly) and hair-hair constraints (That keeps the hair generally together without forcing it to move as a single mass).

More can be read here.

As animation studios such as Pixar strive to produce better works as they progress along, it is only natural for them to update or create new simulators, tools, etc. instead of sticking to present ones that clearly won't work for their new project. 

While there isn't exactly any easy way to animate hair, 3D definitely has the advantage and wider range of choices when it comes to that area, and they are after all the ones that have the tools needed to create ultra realistic hair (Which is something that still can't be fully or as effectively achieved in 2D). 

And so it is neat to see how these softwares can also be modified and improved overtime, all the work that goes into developing them is totally worth it too.