"Simplexity" is a term coined by the creators of the Disney/Pixar film Up to describe the film's animation style as "the art of simplifying an image down to its essence. But the complexity that you layer on top of it-in texture, design, or detail is masked by how simple the form is". Basically, the film was animated with the mindset of making the basic designs of the characters abstract and cartoonish while adding enough detail to features like skin and hair to maintain believability.

So much of modern animation tries too hard to make images more realistic that they come across as dull. One of the chief offenders of recent years is Robert Zemeckis with his work on motion capture in such films as The Polar Express, Beowulf, and A Christmas Carol. His "mo-cap" films have a remarkable degree of detail and realism, but they don't take advantage of the surrealism that animation makes possible. What Zemeckis seems to have failed to realize is that just as animation opens up new possibilities, there are things in live-action that simply don't work in animation. Furthermore, if a computer-generated world strives for as much realism as Zemeckis seems to be trying to convey, the use of animation seems somewhat pointless.

Pixar's approach in Up proves to be far more successful, as they manage to capture the basic key elements of an image while remaining believable. The important part is that Up strives for believability rather than realism, and that's why it works.