Lesson 1: The Basics

In After Effects, an expression is a Javascript based script in that can either modify a parameter or drive the parameter entirely, like position, opacity, or perhaps a Gaussian blur level. We can add an expression to any parameter that has a stopwatch icon next to the parameter. The exclusion to this rule are Masks parameters like Mask Shape, Mask Feather, etc. The key to getting the whole AE expressions thing is to start learning the lingo. So, from time to time, I'd like to try to cover some of these expression terms to get you out of using the pick-whip, and using your noodle to create your own.
Let's start with opacity.
To create an expression in After Effects, you simply hold down the alt (Win) or option (Mac) key and click the parameter. After Effects is then waiting for you to enter brilliant code to tell this parameter what to do. So, let's try this.
Create a solid layer, hit "T" to show its opacity, then opt/alt click the Opacity stopwatch. Now, the text field, enter:
5
Opacity then becomes 5. Pretty simple, huh?
How about:
5 + 5
Opacity then becomes 10.
Expressions are aware of just about any kind of math you want to throw at it.. including + - / * and other weird things like modulus % and even trigonometry, but that's WAY down the road.
Let's throw in another expression term: time. Any time we use the word "time" in an expression, time will be equal to the current time of the composition playback head in seconds. So, if you play your comp from the beginning, time will start at 0 and increase 1 for every second that passes.
If we add this expression to opacity:
10 + time
Opacity will start at 10 and increase by one every second. Note that once the values go beyond 100, the value remains at 100.
Let's try multiplying.
time*100
Things were moving pretty slow before. This picks up the pace quite a bit. We are simply using time multiplied by 100 as the value for opacity rather than using keyframes.
What if we wanted this to start at 5 seconds? There are more elegant ways to do this, but let's do this the easy way:
(time * 100) - 500
Time keeps moving on, ya know. So, at 5 seconds, (time * 100) will be equal to 500. Again, opacity treats any value over 100 as 100 and any value below 0 as 0. So, if we subtract 500, the opacity will not start changing until 5 seconds.
One thing that I don't want to skip over is the parenthesis. Remember back to basic math class... mine was with Mrs. Fleeger. There's an accepted way that we calculate things. Often people use the term "BEDMAS" to remember this. This means that the order we calculate our equation is in this order: brackets (or parentheses), exponents, division, multiplication, addition, subtraction.
The reason I bring this up is that in the above example, we have some parenthesis. The items inside the parenthesis will get calculated FIRST. Then that calculation will have 500 subtracted from it. For example, at 3 seconds we would have:
(3 * 100) - 500 = (300) - 500 = -200
As I mentioned, negative opacity values are treated as 0. We cannot have a negative opacity obviously. This value will continue to be negative until we reach 500 inside the parantheses, which is 5 seconds.
Absorb that for now, and then next time I'll cover Array values like Position and Scale. Don't know what that means? Then check back.