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There are many times when you will want one part of your character to pass in front of, or behind, another part of your character. A good example of this is the hands. A lot of times you'll want your hands in front of your torso, or behind, and so what we're going to do is use expressions to allow you to switch between hands in front, and hands in back. So let's take a look at the character that we have right now. If I select this Left Bicep, and rotate it, you can see that right now this arm and the hand is passing in front of the body, and the reason it's doing that is because, well, it's just a layering issue.
The Left Hand, and the Forearm, and the Bicep are all in front of the Torso. So if we want this hand to be behind the torso, we need to make a copy of it and place it behind. So I am going to go ahead and select my Left Hand, and then I'm going to do an Edit > Copy, and Edit > Paste. So, basically I've just duplicated that hand, and you will see that that layer comes in right on top of the first one. And the reason it's different; you can see that this one is not linked to anything.
But let's go ahead and just right-click over this, and Rename that Left Hand to LEFT HAND 2. Now, once we've renamed it, let's go ahead and link this hand to the original. So I want to make sure that LEFT HAND 2 follows along with the original Left Hand. And then I am going to grab this layer and just drop it right to the bottom of the stack. So now what I have is I have one hand in front, and one hand behind. So if I rotate that bicep and bring that hand in front of the body, you can see that right now it's in front, but if I turn off this Left Hand layer, it reveals the layer behind, and it looks like it's behind.
But we really can't just be turning layers on and off. That's something we really can't animate, but we can animate opacity. So all I have to do is drop the Opacity to 0, and it reveals that hand, but if we zoom in really close, you will see that, well, it only changes this part. We still have three other parts of this arm that need to be placed behind. So we have the Cuff, the Forearm, and the Bicep, and all of those need to go behind the body when we do this switch.
So let's create some additional copies so we can do the same thing with these other parts of the arm. So I am going to go to my Bicep, and I am just going to change my Rotation back to 0, and get it back to my neutral position. So let's go ahead and do some copies. So I am going to go ahead and select my Bicep, do Edit > Copy, Edit > Paste. It's renamed it to Bicep 2, so I am just going to go ahead and do the link. Now I am going to do the Left Forearm, and this time I am just going to use Control+C or Command+C, Control+V or Command +V, and then link it in again.
So I have got Bicep 2, Forearm 2, Copy, Paste, Cuff 2, and again, just link it. So now that I have all of these, select each one of these; Cuff, Forearm, and Bicep, and again, just drag those to the bottom of the stack. I want to make sure that Left HAND 2 is at the very bottom. So now that I have this, I can go ahead and select my Left Bicep and rotate it, and now we can start working with the opacity.
So if I want to take the Opacity of my Left Hand down, I can. I can also do the same with my Left Cuff. But if you notice, now I have got four opacities to manage, and that's starting to get a little bit complex. So what we can do is we can use expressions to help tie it all together. Easiest thing to do is to just add an expression to opacity. So I am going to start with the Left Cuff, select Opacity, and we are going to add an expression. So we are going to do Animation > Add Expression, and then I am just going to use this Pick Whip here to drag it over the Opacity of he Left Hand.
Here we have the Opacity of the Cuff is equal to the Opacity of the Left Hand. So now when I take the Opacity of that Left Hand and bring it down, you will see that the Cuff follows along; very nice! So let's do the same thing for the Left Forearm and the Bicep. So I am going to go ahead into Transform > Opacity > Add Expression, Pick Whip to the Opacity of the Left Hand, and do the exact same thing for the Left Bicep.
Add Expression, and then just connect it all together. There it is. Okay. So now when I take this Opacity of the Left Hand down, the whole arm goes behind. That's wonderful! But we can actually make this a little bit easier, because we can get stuck at a point where it's at halfway between front and back, and we really don't want that. So let's go ahead and make it so it's either one or the other, and we can do that by adding in what's called an expression control.
So I am going to select my Left Hand, and under Effect go to Expression Controls, and here we have a bunch of controls that we can use. In this case, we want to use what's called the Checkbox Control, and basically all that does is it adds an effect here. If we open that up, you will see we have Checkbox Control, and there's really only two values for this check box: On, and Off. Okay, so before we actually do anything, let's go ahead and rename this. I am going to right-click over Checkbox Control, hit Rename, and I am just going to call it Front/Back, and that gives me an idea as to what it does.
And then all we have to do is connect the Opacity of my Left Hand to this checkbox. So again, I'm going to select my Left Hand Opacity, Add an Expression, and then use my Pick Whip to bring it up to the checkbox. When I do, that you will see that when it's off, the hand is behind, but when I turn it on, it seems like nothing happens. That's because we have a little bit of a disparity here; the checkbox gives you a value of 1 or 0.
Opacity goes between 0 and 100. So if you take a look at this checkbox, when it's On, my Opacity is at 1%. When it's Off, it's at 0%. So it's working, but we need to multiply it in order to get it to bring the Opacity up to 100. So in order to get 1 up to 100, you have to multiply it by 100. So I am going to go into the Expression here, under Opacity, and just go to the very end of that, and hit the asterisk, which means times 100. So everything there, times 100, and then just click off of that, and it should work.
So now we have got it on, and off. So now when this checkbox is On, that means the hand is in front; when it's Off, the hand is in back. But we still have one more problem, and that's which hand we are using. You can see that this Left Hand has a time remap on it. So when I change it, the hand behind doesn't change along with it. We still need to use one more expression to connect the second hand to the original. So I am going down to the very bottom of the stack, go into LEFT HAND 2, and you will see we have the Time Remap; it's just not connected.
So I am going to select my Time Remap, add an Expression, and again, just use this Pick Whip and drag it up. Find the Left Hand Time Remap, and just click there. So now LEFT HAND 2 is following the original Left Hand. So when I do my hand, it all follows along, and then when I go behind and in front, I have the exact same hand. So now all I need to do is to lock my copies, because they are aligned exactly with my original layers, and I don't want to mess that up.
So I am going to go down and select Cuff 2, Forearm, Bicep, and LEFT HAND 2, and make sure that those are locked, and now I should have an arm that rotates fairly well. So I can go in front of the body, and if I turn this off, I can go behind the body as well. So as you can see, we've used expressions to create a hand that goes in front of, and behind, the body. The way that we did this was we made copies of the hand, put them behind the body, and then used expressions to tie together the opacity, as well as the time remap on the hands.
So you can start to see how even with simple expressions like this we can get a lot more functionality to our characters.
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