Last year, I went to go see—as many people did—Disney’s now-Oscar-winning animated feature Big Hero 6. On top of being a fun movie, I was really taken aback by how realistic everything looked. I mean, obviously the characters looked cartoony—as is the Disney way—but more than that was how they looked like they actually took up physical space, the backgrounds and inanimate objects even more so. Why does Baymax in his red battle armor look so like it actually exists? Well, the answer to that is especially nerdy.

The Walt Disney Animation Studios put the above video on their YouTube channel and, in the style of an old filmstrip—which Disney used to produce a lot of back in the day—the narrator explains how they go about making things look realistic. The short answer is: light and surface. The long answer is, well, the long answer is what the nine minute video above is all about.

Basically, in any given scene, the animators have to decide what the light source is and where it’s coming from. For something like the sun, which lights the entire surroundings, it seems easy, but it’s actually very complex. You see, the sun isn’t lighting everything the same all at once. Things that face away from the sun but are still visible in light because of the sun’s rays reflecting off of some other object. So, the computer has to treat each individual ray of sunlight as a separate source.

The animators also have to decide what the surface of any given object is because that dictates how the light reflects and where it goes next. Is it a smooth surface or a bumpy one? Is it shiny or dull? These are of utmost importance to track the path of a particular ray. The video also discusses how their computers are able to calculate so much so quickly, but I’ll leave that bit to them to explain.

So the next time you watch a Disney computer-animated movie, just think about how many individual beams of light are touching the object you’re looking at, and then how many beams of light are touching you in the theater, and then stop thinking that because you’re at the movies and should be paying attention.

Does your brain hurt? Tell me about it below!

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HT: Engadget