For some time I've been wanting to put together a document that will assist anyone that wants to learn to write expressions for After Effects. I've been chipping away at it, off and on, for months. I thought it might end up as a book, but I really wanted it to be a resource where you could see the expressions in action and to be able to cut and paste them into your own projects. That caused me to favor an electronic medium. I thought that it might end up as an "e-book" (and it still might), but it has taken so long to get this far with it that I decided to just put it up on my site in the hope that people will find it useful.
Please keep in mind that this resource is a work in progress. All of the sections need more work and there are many more sections that need to be written. Unfortunately, I haven't been able to dedicate the time to this project that it deserves so I decided to just post what I have and add to it as time permits.
The Ground Rules
There are some prerequisites and assumptions associated with this site. First, parts of this site have some movies embedded that are possibly large enough to make it a frustrating experience if you're using a dial-up connection. I'm assuming that most people that visit here will have access to a broad-band connection. If you don't, I sincerely apologize.
Next, this is not an intro to After Effects. I'm assuming that you already know your way around After Effects. So if you run across something like "create a comp" or "keyframe the opacity changing from 0 to 100% over 3 seconds" or "make the body the parent of the arm" I'm assuming you know how to do these things. If that's not the case then you probably need to hook yourself up with one of the fine books on After Effects (especially Volume 1 of Trish and Chris Meyer's "Creating Motion Graphics with After Effects") or the wonderful series of videos from Total Training and come back later.
I'm also assuming that when I say "Alt+click" you know that I mean to hold down the Alt key and left-click with the mouse.
At this point feel I need to apologize to Mac users. I don't own one, I don't think I've ever even touched one. So rather than risk making the wrong translation, I'll assume that you can translate "Alt" to "Opt" or whatever's appropriate on your system.
I'm also assuming that you're using After Effects 6.0 or later. However, most expressions, if they don't use a feature that is new to AE 6, can be easily converted to AE 5.0 or 5.5 syntax. In most cases you just need to convert things like "thisComp" and "frameDuration" to "this_comp" and "frame_duration". If you don't happen to find them all, After Effects will provide you with a helpful admonishment such as "Class 'Layer' has no property or method named 'frameDuration'". Better idea - upgrade.