After Effects expressions: Comp, Footage, and Layer attributes

Chris and Trish Meyer posted the 5th of a 12-part series, Deeper Modes of Expression, based on an extra in their book Creating Motion Graphics with After Effects. The fifth installment is Deeper Modes of Expression, Part 5: Comp, Footage, and Layer Attributes:

"If you can keyframe it, chances are you can control it with an expression, or use it as part of another property’s expression – usually by dragging the pick whip to it. However, there are additional properties you have access to – including those you can’t access with a pick whip, such as whether a layer’s audio or video switches are on, or its current in or out point. You’ll need to either manually type in code to access these, or get some help from the expression language menu. To give you a taste for what’s possible, let’s discuss a couple of comp and footage attributes of interest." Read more...