Expressions

[Filip Vandueren]

Well,

we use expressions whenever we think the result is easier to calculate than to achieve by setting keyframes.
But it\’s important to remember that\’s all it is: a calculation.

As to what/how to write them, it could be as simple as using the pickwhip to reference one property\’s value to another to copy it ( when for example two layers should always have the same rotation )
or as simple as a bit of math. time*3 is a calculation that yields an evergrowing number that increases by three each second. Handy for evolution of noise for example.
These simple examples are mentioned in the manual, and also in other After Effects books like the ones by Chris and Trish Meyer.
But it can be as complex as a 200 line little program that at it\’s end -again – just results in a number.

The basis for all this is JavaScript. There are plenty of sites out there with free tutorials on the basics of js as it pertains to the webbrowser, as wellas books. I still think there\’s no book that specifically caters to motionscript (the dialect of js used in after effectx expressions) though there are a few great sites like motionscript.com.

So how would you know what to write?
Get a grip on JavaScript, and see if you have a knack at breaking things down into manageable computable chunks.
Once you understand that JavaScript is object oriented, check the After Effectx manual to find out what specific objects and methods After Effects has extended JavaScript with to access the values in it’s layers, effects, comps etc.
Don\’t run before you can walk. Ask questions ( specific and to the point), mentioning that you\’re eager to learn, so we wil elaborate a bit on code we suggest.
And since you’re new to After Effects, not just to expressions . Maje sure you first understand the app itself.
The basics forum ( and the manual !) are your friends.

[Todd Kopriva]

I think that Filip did a good job of summarizing. There are a few good online resource for learning expressions. I find that that it’s useful to work my way way through some examples when learning a new language or set of tools.

I think that you’ll find that people on this forum (especially Filip and Dan) are very good about answering the specific questions that come up as you’re learning, as long as you’re also putting forth the effort to teach yourself.

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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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[Bert Brown]

https://motionscript.com/mastering-expressions/table-of-contents.html

an oldie, but goody. this was a great place to start, for me.