POWER! A lot of POWER! That’s what it takes to render
your fave CGI,CGA movies. The 55,000 core supercomputer used to render Big Hero 6 delivered 1.1 million computational hours PER DAY!
Toy Story used 800,000 computational hours TOTAL over about a year!
In order To give you a sense of scale, the first 10 min of Brave consumed a massive amount of computational power. Merida, the main protagonist of the movie Brave, has red curly hair, the horse she rides has a flowing mane, the organic backdrop of the forest, water, waterfalls, grass, trees etc. Only when you realize in CGI and CGA Nothing is FREE, the animator must place EVERY object, AND how that object interacts with the scene, in MIND crushing detail, it then becomes clearer what takes all that POWER. Here is a link to a mathematics paper published by Pixar JUST ON THE MATH OF THE PHYSICS OF CURLY HAIR. http://graphics.pixar.com/library/CurlyHairB/paper.pdf . Every single strand of hair is: simulated, computed, drawn, textured, illuminated and rendered for the final frame. Below is the formula used to simulate JUST ONE type of Springiness of a SINGLE curly hair for Merida.

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The final single frame rendering takes 2-4 hours of SUPERCOMPUTER time on average, some frames taking DAYS. Knowing what it takes to make hair lifelike, you will Shutter at what it takes to make LIGHT lifelike. The computational power used  to make Big Hero 6, a movie that broke new ground on the physics of light refraction, totaled 199 million computational hours, and that system would be able to render ALL of the Movie Brave in 10 DAYS.

A frame From Pixar's Brave

This one took some time!

A frame from Pixar’s Brave 2012 Copyright