Long hair is theoretical.
It takes 5 to 6 years to make a Pixar movie and computers are involved close to the end of the creation process. Technology, that creates the around 250 billion image dots (pixels) of an animation movie, is a tool in the hands of the artists.
In his painting creation analogy Rob Cook thinks that technology in animation movie creation is used like a painting brush.
There are lots of previous steps that must be followed as with any normal movie. Starting from the creative step of story telling by the director followed by the visual story telling by the story artists to casting, voice recording, editing, etc.
Computers are involved in creating the 3D models, layout, animation, simulation, shading, lighting, special effects and images rendering.
Reproducing reality is not trivial. This is because, in my opinion, the laws of physics must be accounted for, especially gravity and light effects.
If you look at a cartoon or most of the video games you realize that there is something missing in the animated characters that doesn't give them the feeling of being real.
It was puzzling for me to learn that 90% of the computer simulation efforts put into reproducing realism like effects is in simulating the clothes and hair of the subject in motion.
Mathematical models (like Finite Element Analysis) are used to simulate the stretches and stresses clothes are subject to while subject moves.
Moving hair simulation is another challenge. A challenge sometime so big that in the case of long hair simulation, made the Pixar team temporarily conclude that simulating "long hair is theoretical".
Every second of an animated movie requires lots of effort. A well rewarded effort if the movie makes our virtual life experiences richer.
1. Animation World Magazine: http://mag.awn.com/
2. Papers by Rob Cook: http://graphics.pixar.com/library/indexAuthorCook.html
3. What's Pixar's RenderMan: https://renderman.pixar.com/products/whatsrenderman/index.htm