omino pixel blog

I was going to write a short post about using After Effects expressions to capture the “feel” of machines. But I got so distracted by this simple example I put together:

(A minuscule validation of this simple animation is that just now, as I’m typing, my cat is intently watching it loop in the window behind. We get it where we can!)

The behavior comes about naturally; the big wheel has its rotation controlled by the little wheel’s rotation divided in half. And the little wheel, in turn, is controlled by the y-position of the blue slab.

What struck me was the accidental (but obvious, now) trick of exposing an offscreen action with onscreen elements. We “know” that the wheels are associated with the blue slab’s motion. So when they spin for a while, we know it’s rising and, momentarily, falling.

So what happens if you break that assumption? I guess it is “surprising” or even, just maybe, funny…

I know this stuff is all trivial basics to a skilled practitioner of the visual or animation arts. But I’m not one, so it’s fun to rediscover bits and pieces along the way.So anyway. About expressions. The wheel is controlled by the blue slab height. That way I could keyframe its motion to give it the desired motion and heft. At first, like an elevator, and later, like a sixteen-ton weight. But I wanted the wheels to feel like the pulley system. So although they are in fact controlled by the blue slab, they track its motion 2/10ths of a second — fully six frames — in the future. Here’s the rotation expression:

thisComp.layer("Cyan Solid 1").transform.position.valueAtTime(time+.2)[1] * 4

You can link together a handful of elements’ motions and effects to get arbitrarily nifty and complicated machines! Here’s a sort of microscope, where expressions are used to control a simulated rack focus effect.

Mmm!(Download self-contained After Effects project here.)