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Expressions are just another way to control properties of layers in After Effects. Now they look like a programming language and they are kind of our programming language, but they are not that challenging. So, a lot of new users are intimidated by expressions. You need not be. So, let's go ahead and open up this Left Wheel layer properties. Go ahead and press T for Opacity. The way we set expressions is by holding down the Option key on the Mac or the Alt key on the PC when we click the stopwatch. Then we get this field that allows us to input information.
So, it's just looking for a number here. That's basically all expressions are. I could type in 75 and if I press Enter on the numeric keypad or simply click outside of this field to accept that value, then that's what happens. It just turns the Opacity to 75%. So, as you could see here, Opacity, the value turns red and the red indicates that the value here is being received or calculated based on the expression. So, I might say 50+20, and I click away.
Well, if we take the Right Wheel, for example, make this the child of the Left Wheel and if we select the Left Wheel and hit R for rotation, when we rotate the Left Wheel, you would think that they would both rotate in the same way. But they don't because you use the parent's Anchor Point. So if we rotate the child by itself, if I press R and rotate the child, the child still rotates on its own. But when the parent rotates, the child rotates around it, it rotates around the parent's Anchor Point. So, actually what I can do here is remove the parent relationship by going to this dropdown and choosing None, and then I am going to actually zero out these Rotation values and what we can do here is create an expression.
Now actually I am going to undo this a few time. We just notice that our Right Wheel here is up in the air. Once you take away the parent-child relationship, then you got to be careful of where you're at, at that time, because that is how the objects will stay in that condition. So, I am actually going to zero out Rotation, then remove the parenting relationship. Sorry about that. I had to undo a few times. But I want to create an expression now that will connect these, basically making it so that the Right Wheel will get its value from the Left Wheel's Rotation.
So, they are not really connected on a layer basis like parenting. Just these two properties will be connected. So, I'll go to the Right Wheel. I'll Option+Click or Alt+Click the stopwatch here for Rotation. Instead of putting in a number here manually, I'm going to take this pick whip again. I am going to click and drag to the Rotation property of the Left Wheel layer. That will basically say that in this composition there was a layer called Left Wheel and in that layer Left Wheel, there is a transform called Rotation.
So, basically make this value whatever this Rotation value is. So, I click outside of that and now if I rotate the Left Wheel, the Right Wheel goes along with it. So, if I drag out here, let's say, to 79 degrees. You'll notice as soon I let go off the mouse that the Right Wheel, its Rotation is 79 degrees as well. So, really then, instead of just typing 79, it's getting a live update from this layer's Rotation value. So, as you could see, again, expressions do not need to be overly difficult and complex to be very useful.
Let's look at another example of how we can make expressions work for us. Now we've been looking at this Bike girl throughout the entire training series and in the beginning I went ahead and did this for you. I connected her feet to these pedals using expressions, because I knew that later on, right now in the training series we'd be talking about how to do this manually. So, as you could see now the Top Pedal and the Bottom Pedal are no longer connected to the feet. So, as I move in time, there is no connection between the pedals and the feet. So, what we're going to do is we're going to connect these pedals to the feet.
Now the feet don't really have a point or anything to them, like if you have a certain layer, there is not like the back of the layer with the left side of the layer. It's just a layer. However, I did-- if we select the Biker Body layer and press U two times-- you'll see that we did add the Puppet tool to this. So, what we can do is link the position of the pedal to one of the Puppet control points and that will get us in the ballpark of having linked the pedals to the feet.
So, what I am going to do is find this point right here, which is actually Puppet Pin 1. So, if I scroll down right here, you'll see that it is now selected here in the Composition panel and also here in the Timeline panel. So, what I want to do is I want to control the Top Pedal's position by the Puppet Pin 1. So, I am going to select P for position with the Top Pedal selected. I am going to Option+Click or Alt+Click the stopwatch to get an expression and I am going to pick whip to the Puppet Pin 1 position and let go.
Now I could accept that by clicking away from the layer. And we see now that as we move in time this pedal has been connected to the foot. So, we're seeing it right below there. It's a little bit pixilated because we're zoomed in above 100%, but you can still see that there is the pedal underneath that foot. We could do the same thing with the other pedal, but the Bottom Pedal is actually connected to the Biker Right Leg. So, we can go ahead and press U to reveal all of the keyframes for the Biker Right Leg, and we might want to select one of these pins, and actually if I click this one, we could see that it's Puppet Pin 1 on the Biker Right Leg.
That's the pin we want to connect to. That's the foot pin. So, we could go to the Bottom Pedal layer. Press P for Position. Again, we'll create another expression. Alt+Click or Option+Click the Stopwatch, and then drag the pick whip to Puppet Pin 1 on the Biker Right Leg layer to that Position property and click away to accept it. And now as we move around here, we could see that our pedals are connected to the feet pretty much, and that gives you illusion that she is actually riding this bike.
We'll talk more about expressions in the next couple of movies, but basically, what I want to convey to you about expressions now in this movie are two things. Number one is that, really expressions are just trying to get to a value. They are just trying to come up with a number in a creative way. And it's basically a way to control a property of a layer. And number two, I wanted you to be able to see how to connect different properties using this pick whip.
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