Friday, July 28, 2006

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:


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.


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.


Blogger Beans said...

This is so Sweet!

Thank you so much for starting an expressions blog! I can't wait to delve into the world of expressions as this is the final frontier for my learning curve!

6:21 AM  
Blogger espi said...

This comment has been removed by a blog administrator.

3:28 PM  
Blogger espi said...

niiice!! dude i've been waiting for someone to explain all this expression stuff. I did all the tutorials and stuff moved. I just didnt understand why! Look forward to your blogggg

3:33 PM  
Blogger Michele Yamazaki said...

Mrs. Fleeger. Oh, I remember her. Mrs. 'Flea Bag' ;-) Actually, I never had her, but I heard rumors about her that she was a witch. Poor lady.

I'm going to study your expressions so I really know what I'm doing. My math education was not the best (maybe because I didn't have Mrs. Fleeger!) so I'm a bit weak in that area. Well, not THAT weak ;-)

12:04 PM  
Blogger taliban said...

you rock!!! i've always wanted to get into expersions. you explain beautifully. can't wait for more lessons. (i feel like such a geek)

12:58 PM  
Blogger astep said...

Thanks for doing all the work for me Harry!

PEMDAS....."Please Exuse My Dear Aunt Sally" was what Mrs. Powell came up with. All this time I have been calculating the multiplication before the division.....thanks alot Mrs. Powell!!!

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Blogger George Rodriguez said...

These are cool!

6:56 AM  
Blogger Inerxia said...

Eres mi AE heroe !

Thank you !

10:15 AM  
Blogger Serial Dancer said...

Thank you for taking the time to explain expressions!
your the best!

9:00 AM  
Blogger Antonis said...

Thank you for your time!
I greatly appreciate your lesson!!!

7:41 AM  
Blogger Armybert said...

cool, im learning!

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Blogger chenna said...

i am learning

1:05 AM  
Blogger Sarah Chalek... said...

Thanks you! I'm trying to learn After Effects for my job and I was really confused about expressions until I read this. I'm going to read the rest of your posts now.

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Blogger arun said...

Check out the AE Scripts video tutorials at


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Blogger Michael said...

I am so glad to have found your blog. It's a great source for building foundation knowhow about expressions! Thank you!

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I am so glad to have found this blog! Thank you for your work!

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Thank you dude

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