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Expressions are one of the most powerful but underused features in After Effects. They can be used to animate layer properties with code, as compared to explicitly keyframing every value in the Timeline, and have multiple parameters and layers that follow the lead of a master layer or controller effect, making it much easier to coordinate complex animations and quickly accommodate client changes. In this introduction, Chris Meyer shows how to let After Effects do most of the work by creating simple but very useful expressions that can be put to work on a wide variety of jobs.
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Let's keep building upon what we have learned by using the Pick Whip tool and some simple math to create an animation which would otherwise require a calculator and a little bit of thought. First, to clean things up, I am going to go to the Comp panel and select Close All, just closes all compositions, go back to the Project panel, and open up the comp 03-Clockwork starter. Here we have all the pieces of an old watch face. The minute hand had already been keyframed, and I'll press 0 to RAM Preview. You'll see it's doing a nice sweep around the screen.
The hour hand and the second hand have not been animated yet. I'll press R to reveal the Rotation. And, by the way, for those who watched the After Effect Apprentice lesson on parenting, the anchor point is indeed very important for the rotation of things like the hour and second hand. That way when we animate the rotation, it is animating around its pivot point, not just the center of the layer. Let's say we want this to work like a real watch, where the hour hand goes slower than the minute hand and the second hand goes faster than the minute hand.
Let's start with the hour hand. First, I'll make sure the rotation parameter, the parameter I want to work with, is already exposed for these two layers. I'll hold down Option or Alt, click on the stopwatch for rotation, use the Pick Whip tool, and drag it up to Rotation for the minute hand, so now they're tied together. However, the hour hand goes slower than the minute hand. When the minute hand has made one full rotation, the hour hand has only moved one twelfth, one hour around the dial. Well, with expressions that's very simple.
I'll type slash for divide, 12, divided by 12, go one-twelfth speed. Press Enter on numeric keypad, press 0 to RAM Preview, and you'll see that the hour hand is indeed moving at the correctly matched space to go along with the minute hand's animation. Perfect. Let's tie together that second hand as well. I'll hold down Option or Alt, click on the animation stopwatch for rotation, drag the pick whip back up to the Rotation for the minute hand, and in this case this second is supposed to go 60 times as fast as the minute hand, 60 second in a minute.
So I'll do asterisk for times, 60 for sixty, and I will just click off somewhere to accept the expression. Now as I step through the animation, you'll see that the second hand is indeed jumping ahead. I can turn on Motion Blur for the composition-- we've already turned it on for the individual layers--to see a little bit more fun movement going out to that second hand. Press 0 to RAM Preview. The second hand is really roaring at the speed of the minute hand, but you get an idea. Now, let's remember that nasty client who keeps changing his mind.
In this case, he may say, "That second hand is going way too fast. Slow down the whole animation." Well, if you've done this all with keyframes, you'd have to reenter new values for the minute, hour, and second rotation to the get them all to line up. But in this case, we'll just go that last rotation keyframe for our master, the minute hand. We'll slow it down to maybe 0 rotations and just 120 degrees. Hour and second hand is automatically updated, RAM Preview, and now we have an animation that's much easier to follow.
And what's most important is we edited just one keyframe and everyone else followed along. I know I'm really hammering this point home, but that's why it's worth learning expressions. Ultimately, they will save you a lot of time, prevent you from making errors, and allow you to get to bed earlier, rather than being up all night accommodating client changes. So we got pretty far here just by using the Pick Whip tool and some simple math; however, you can take your expressions skills to the next level with just a little bit more work.
That's the subject of the next chapter.
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