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Let's build upon what you learned in the last movie to use expressions to tie together similar properties. In this case, I'm going to open up the comp 02-Simple Math*starter, select both layers, Command+A or Ctrl+A, press U to reveal their animating properties. And once again, the first letter has been keyframed the second layer has been tied the first layer using expressions. I'll do a quick RAM preview here. Nothing strange there. Okay, let's say we want a jazz this animation up, add a drop shadow to these layers.
I'll select the blue pulley and apply Effect > Perspective > Drop Shadow. And to make it stand out a little bit, I'll give a little bit more Distance, maybe just a little bit Softness. That looks nice. Now if you've watched the previous lesson in our After Effect Apprentice series on nesting and pre-composing, you know that we drove home the point of the render order of layers, and by looking at the timeline panel, you can quite often figure out what that render order is. In this case, effects such as the drop shadow happen before the transforms, such as the scale and rotation. And indeed, as that blue pulley rotates, you can see the shadow, unfortunately, is rotating along with the pulley, which is not at all what we want.
Now there are a few ways of fixing that, including by using expressions. Rotation is a very simple parameter using degrees. The Drop Shadow also has a Direction property that uses degrees. What can we do with expression to tie these two together? Let's think for a second. As that pulley rotates counterclockwise, we need the shadow to rotate in the opposite direction by exactly the same amount to stay in the same relative position.
Okay, that's gives me the clue here: before you apply expressions make sure that all the properties you want to deal with are exposed. My Drop Shadow is exposed in Effect Controls panel and the blue pulley's rotation is exposed on the Timeline panel. That's good. We need the drop shadow to follow the rotation, so the drop shadow is what needs to get the expression. I'll hold down Option on Mac, Alt on Windows, click on the stopwatch for direction. It'll appear down here in the Timeline panel with expressions enabled.
You'll see that it has a red value now. Its text is highlighted, ready for action. I'll click on this pick whip and drag this pick whip down to Rotation property for the same layer, and you'll see it's picked up the transform rotation already. It wrote the code for me automatically. Now if I leave this alone, it just means it is going to be the same, but what I really need is for this to rotate in the opposite direction, times -1. Press Enter, and now as I scrub through the timeline, you'll see the shadow is appearing to remain stationary.
In reality, its direction is just rotating exactly the opposite to the rotation of the gear. Well, we are most of the way there, but now my shadow is going up the screen, not down to the right like it was before. That's easy to fix. Remember, you can also add offsets these layers. I can type in a number, like say 120 degree, or remember, you can say +value to pick up the underlying value of this property, which defaults to 135 degree. I'll press Enter.
My shadow falls down in position. I'll turn off expressions briefly to remind us that we did indeed have 135 degree underneath, and here is our lovely shadow now exactly where it's supposed to be, thanks to expressions. Expressions, by the way, are just another property of an effect or other value. For example, if I select Drop Shadow, Command+C or Ctrl+C to copy, go to the Red Pulley and Command+V or Ctrl+V to paste, not only will it paste the drop shadow, it will paste the expression attached to that drop shadow.
You'll see that it has a red value now to show that it has an expression applied, and now both of these layers indeed have properly positioned drop shadows. So that's another use of expressions, just to solve some very simple math issues of things rotating the wrong direction, rotating too fast, too slow, et cetera. I know math is usually an ugly word for artists; however, by combining expression with just a little bit of math, you can solve a lot of problems without having to add extra keyframes or come up with another design approach or whatever.
Expressions can be your friend. In the next movie, we'll take this to the next level by tying together the different hands of a watch.
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