Expressions

Music is math.

Michael Sandison and Marcus Eoin (Boards of Canada)

Expressions extend the functionality in After Effects by enabling you to link elements of your project together and apply logical and mathematical relationships.

Did your eyes just glaze over? 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.

I don't buy it: As an After Effects artist, you already deal with this stuff all the time. If you're not making full use of expressions, you're missing out on a more elegant approach that can eliminate tedium.

Eliminate tedium? But, you reply, expressions require the use of code. What could be more tedious to a visual artist than managing code?

This chapter makes the case that it's worth dealing with a little codeoften just one line in these examples, all or part of which can be entered automaticallyto attain the flexibility and power of expressions. The examples given here relate directly to other techniques explored in this book. They are practical, not theoretical.

To give you a taste of what is possible with expressions, here are some of the case studies explored in this chapter:

• Create an automatic one-to-one relationship between data from two animation channels (whether from the same layer, separate layers, or even separate compositions)
• Scale and offset property values
• Link values to a slider control for easy access
• "Mute" keyframe values by replacing them with a constant value
• Loop, with several different options how you do it
• Destabilize or smooth camera moves or any animation data (without affecting or destroying the data)
• Use time and index values to progress animation data
• Create a conditional statement that causes something to happen when specified criteria are met
• Trigger actions to occur automatically under specific circumstances
• Emulate 3-D tracking in a forward-moving shot

Overall, the goal, as always, is to provide a guidebook rather than a recipe book, enabling you to build on the understanding you gain here by moving from simple to complex examples.