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Now, before we go further, let's show a few ways of managing expressions. This little equal sign is very important. It shows that an expression is active. If I click on the equal sign, it becomes not equal, and that expression is now temporarily disabled for that layer. I will RAM preview, you will see I still have my scale animation; I just no longer have the rotation animation. But if I want it back, I will click on this again and turn that back on, and now that expression is in force again.
If I indeed want to delete the expression, I can either select the text and Backspace to delete it, I will undo; or again, Option+Alt click on the property to again delete that expression. I will undo again. Let's say that you want to reveal the expressions applied to a layer. Well, the same key that you used to reveal keyframes, U, will also reveal expressions for a layer. I'm pressing U again, and there are my expressions.
So U reveals both keyframes and expressions--anything that's causing animation to happen. Now let's say I happen to have both happening on this layer, keyframes and expressions. I will just press Shift+T to reveal Opacity and just keyframe a little fade-up here for the red pulley. Zero. If I wanted to reveal just expressions, I will select the layer and type EE, two Es in quick succession, and I get just the expressions. If I want to see everybody, I press U once to twirl up everybody, and U again to reveal all the animating properties.
There is no key to reveal just keyframes and not expressions, so sometimes your timeline can get a little bit messy, but hey, it's not bad. The other thing you need to watch out for is what happens whenever you induce an error in expression. Let's say that I accidentally just type garbage at the end of this expression and then click Enter. You will get a warning message whenever you have an expression that does not compute, that does not make sense. Here it says, "After Effects warning: Expression was disabled." And it tells you a bit of detail. It occurred at the first line, the property Rotation is not quite right.
Click OK, and now you will see a little warning symbol right next to the bad expression, and it automatically will be turned off. Just turning it on is not going to fix it; you'll just get the error again. What you need to do is either carefully delete your error in the expression, or in my case I can just undo to get back to where I was. Press Enter on numeric keypad, and now I have an enabled expression again, and life is good. And now I am going to delete my Opacity keyframes for now, just to get back these two layers being equal.
So that's the basics of applying expressions. You use the Pick Whip tool to tie a property of one layer to the same property of another layer. Very simple. Edit the keyframes of the master layer, the expressed layer will follow along. But of course, there is lot more you can do, so let's start to dive into that in the next movie.
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