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In the previous movie, I showed you where you could accidentally screw up an expression by adding some undesired text onto the end of it. Just some garbage characters, Enter, and I get this error message. I'll undo. However, there are times when you do want to edit an expression. For example, you may want to perform some very simple math, like make something go faster, slower, bigger, or smaller. Let's start with rotation. Let's say that I want the red pulley to rotate twice as fast as the blue pulley is.
I will press the asterisk key to signify multiply. Yes, you kind of have to use computer math, not real calculator math here, and then 2 for times 2. Press Enter and you can already see that where the first layer is at 2.6 degrees, my expressed layer is twice of that, 5.2 degrees. I will RAM preview and now that red pulley is indeed spinning twice as fast. Let's say that instead you want the pulley to go half as fast. Well, that's simple.
Rather than asterisk, or times 2, you do a slash or divided by 2. Enter, RAM preview, and now it's doing the same animation, just only half as much. And again, you make any changes you want to the layer that you are following, the master, and the express layer will automatically be updated to follow along. Expressions are live; you don't need to copy and paste all of your changes. I will move this keyframe back later in time.
What if you want the red pulley to rotate in the opposite direction? That's simple enough. I will do *-1. Enter, RAM preview, and now you see that they are now rotating in opposite directions. So this is all very simple. You can do the same thing with the Scale property. Let's say you want the red pulley to be half of the size of blue pulley. You can do divided by 2, /2. Now it's just half the size, but otherwise performing the same animation.
Let's say instead you want it to be 90% of the size, almost as big, but not quite of the same size. Again, very simple, asterisk for times, 0.9 for 90%, and you will see where the first layer is 100%, our expressed layer is now 90%. Preview. Similar sizes but just slightly different, and it goes on and on. I have shown you multiplying and dividing. You can also do simple things like adding. For example, maybe you want the red pulley to have the same rotation, but just to be offset very slightly, say 30 degrees, so that their spokes are not exactly lined up.
Preview, same rotation, just slightly offset to create some appearance of irregularity in the animation. Now, the really nice thing about expressions is that they can be combined with keyframes or the original value applied to a layer. For example, instead of saying +30, I can say +value. Value says add this expression's contribution to whatever the underlying value or keyframed value of this layer happens to be.
I will press Enter to accept, turn off expression for a second. My Rotation value is 0 so it's not very interesting, but if I made it something like, say, 25 degrees, turn the expression on, now you see this pulley is offset by 25 degrees. The really cool thing is that this is even live. You can scrub the value. It will temporarily switch back to showing you the underlying value of this layer. Maybe I'll go negative. Release and now you will see the result of the expression, following the blue pulley, then adding the underlying value of the layer.
And the same would hold true if I was to keyframe this layer, rather than just set a constant value for it. It triggers your memory to tack on "+ value" to the end of most expressions.
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