Chapter10.Expressions

Chapter 10. Expressions

Music is math.

Michael Sandison and Marcus Eoin (Boards of Canada)

Expressions open up a whole new realm of functionality in After Effects by enabling you to link elements of your project together and apply logic and bits of math to the connections.

Here is where the standard disclaimer is usually added about how artists don't like logic and bits of math and where I promise to shield you from it as much as possible. But I don't buy the underlying assumption that because you are a visual artist, you don't want to be bothered with pesky numerical data, technical details, logic, or math.

Admit it: You deal with that stuff all the time. Of course you do, you're an After Effects user. The job of expressions is, generally speaking, to make logical connections easier and to automate the tedious parts of animating. So it's not like you get out of having to deal with this just because you don't fully understand expressionsyou still have the problem, just with a less-elegant solution.

There is one thing about expressions, however, that strikes the majority of After Effects users as inelegant: They require the use of code. To harness the power of expressions, you have to examine, think about, and occasionally type little bits of code.

In this chapter, I will try to make the case that it's worth dealing with a little bit of codeoften just one line in these examples, all or part of which can be entered automaticallyto get the flexibility and power of expressions. If you're still not convinced to at least give it a try, realize that you have condemned yourself to spending more time piecing together solutions in the UI to perform operations that expressions are designed to do.

With that out of the way, let's look at an overview of what is possible with expressions. Some of the most typical things you might do are

• Create a one-to-one relationship between data from two different animation channels (whether from the same layer, separate layers, or even separate compositions)

• Scale and offset keyframe values

• Link values to a slider control for easy access

• "Mute" keyframe values by replacing them with a constant value

• Loop

• Destabilize or smooth camera moves or any animation data (while the source data remains in place)

• Use time and index values to progress animation data and specifically, to remove noise from a locked shot when it is too prominent for the Remove Grain effect

• Create a conditional statement that causes something to happen when specified criteria are met and specifically, to trigger a film dissolve using comp markers

• Emulate 3-D tracking (in case you don't have access to a third-party 3D tracker)

In looking at that list, you may see things that you've always wanted to do in After Effects and things you'd never thought of doing. If you can get the hang of how these examples work, you'll be able to apply the same principles to similar but unique situations on your own.

Rather than taking you step by step through learning expressions from scratch, this chapter outlines and deconstructs useful applications of expressions to demonstrate how they work, moving from the simplest to the most complicated examples. I'll do my best to fill in the gaps for you, and at the end of the chapter, I'll touch on what to do to go beyond what's here.