What causes 3D animated “bloopers”?

Question

I’m talking about things like [Merida’s eyes being all buggy when animating Brave](https://www.youtube.com/watch?v=4405qEhBmEM) or [everyone’s hair going in weird directions in The Incredibles](https://www.youtube.com/watch?v=Y_MQnH54XaA) or [Shrek’s arms suddenly getting really long](https://www.youtube.com/watch?v=UNngNiYXuVE). Why does the animation fail in (apparently) random and uncontrollable ways?

In: Technology

Answer 1

In the *really old days*, 3D animation was sort of like stop-motion animation. The people who made the movie had to do a lot of work to manually describe how each part of the 3D “models” would move between poses. That’s why very early animation avoided things like hair or fur, there’d be too much manual labor involved. Since it was so hands-on, mistakes didn’t often make it to the finished product.

As it got more sophisticated, they started writing small programs to do that work for them. That let them focus more on the “key” frames of the major models and the programs would figure out how to move all the body parts. Meanwhile other small programs handled eye movements or the way hair/fur would move.

But programs can have mistakes. All of the descriptions of how stuff moves is just Physics math on 3D coordinates.

A common mistake in this kind of math is to accidentally have a “something squared” when you just need a “something”. That will turn results that are “supposed” to be negative numbers into positive numbers. That usually means something that was supposed to move a tiny bit one way moves in a different direction and for what may end up being a long distance. For example, if something’s intended destination had the x coordinate -2.8, it will now end up at +2.8 so if it was starting from -2.7 you can see how that’s way off.

So what you’re really seeing is someone making an error in those little programs that help these movies do more complicated things. Often the only way to catch the error is to render a whole scene and watch it very carefully. In a very long movie, sometimes things get missed, or they might just decide it’s too much effort to fix and people won’t notice without looking very close.