Building the World's Greatest Cameraman Using After Effects 5.5 Expression Controls
Dan Ebberts July 26, 2023
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Originally published in 2002
With version 5.5 of After Effects, Adobe added the wonderful world of the powerful new Expression Controls. In this tutorial, Dan Ebberts looks at several different ways that expression controls can be useful in your projects.
This is an Advanced Tutorial.
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introduction In this tutorial we're going to explore the wonderful world of the powerful new expression controls which Adobe introduced with After Effects 5.5. We'll look at several different ways that expression controls can be useful in your projects. First we'll examine the most straight-forward use of expression controls, which is to use them as a manual adjustment master control to help you set up a comp just the way you want it. Then we'll explore using expression controls as master animation controllers. Next we'll examine how you can use an expression control as a limited type of global variable to distribute information to expressions in other layers. Finally, we'll have some fun using expression controls to create the world's greatest cameraman. That's a lot of ground to cover, so let's get moving!
the basics
After Effects 5.5 introduced six new expression controls: slider control, angle control, checkbox control, color control, point control, and layer control. These controls are applied as effects and are located in the Expression Controls group in the effect menu. When using these for global animation control, I like to apply them to a null layer (and then turn off the visibility of the null so it doesn't clutter the comp window) so it will be easy to find them later, but you can apply them to other layers as well. The first control we're going to look at is the slider control.
slider control
Go ahead and open the project file and select the tab for the slider comp. This comp contains a null layer (to which a slider control has been applied), a simple Illustrator graphic layer that has been duplicated a bunch of times, and a camera. Notice that the graphic layers are 3D because we'll be using the slider control to position the layers in Z space. Select the null layer and hit Ctrl Shift t (PC) or Cmd Shift t (Mac) to reveal the Effect Controls palette. Select one of the graphic layers and type p to reveal the position property. Click on the little twirly to reveal the following expression:
position [0,0,(index-1)*this_comp.layer( Null 1 ).effect( Slider Control ).param( Slider )]
This expression establishes each layer's position in Z space based on its layer number (index) minus one, times the value of the slider. So as the value of the slider increases, the layers will spread themselves out along the z axis. Go ahead and scrub the value of the slider (by clicking on the value and dragging). Pretty slick, huh? Notice that if you drag the value negative, the layers spread out in the negative z direction. You can also adjust the value by clicking on the little twirly next to Slider and dragging the slider. One other thing to note is that if you right-click (PC) or Control-click (Mac) on the value and select Edit Value from the context menu, you can adjust the range of the slider. You might want to do this to give yourself a wider range or more precision with the slider.
Note that setting up a comp like this is fairly simple. The graphic layers are all exactly the same. Once you get the expression set up you can quickly create a bunch of duplicates by typing Ctrl d (PC) or Cmd d (Mac) and the expression will automatically accommodate any number of layers.
angle and color controls Select the tab for the angle and color comp . This comp contains a null and multiple copies of a graphic layer that has had its anchor point moved to the bottom and had the tint effect applied. Select the null layer and hit Ctrl Shift t (PC) or Cmd Shift t (Mac) to reveal the Effect Controls palette. You'll notice that the null has an angle control and two color controls applied. The color controls have been renamed start_color and end_color so that we can refer to them by name in our expressions. Select one of the graphic layers and type r to reveal the rotation property and click the little twirly to reveal the following expression:
index*this_comp.layer( Null 1 ).effect( Angle Control ).param( Angle )
This expression calculates the layer's rotation by multiplying the layer number (index) by the value of the angle control. With the graphic layer still selected, type e to display the effects applied to the layer. You'll notice that the tint affect has been applied. Click the little twirly next to Tint and then click the twirly next to Map White to to reveal the following expression (you'll have to expand the expression area to see it all):
if (this_comp.num_layers<3){
this_comp.layer( Null 1 ).effect( start_color ).param( Color )
}
else{
this_comp.layer( Null 1 ).effect( start_color ).param( Color )
(this_comp.layer( Null 1 ).effect( end_color ).param( Color )-this_comp.layer( Null 1 ).effect( start_color ).param( Color ))*
((index-1)/(this_comp.num_layers 2))
}
This expression first checks to see if there is only one graphics layer, in which case it just uses the color from the start_color control (this is necessary because without this code, the expression will abort on a divide by zero error when you first apply it to a single layer). The rest of the expression covers the usual case where there are multiple graphic layers. In that case, the expression spreads the color out so that the top layer gets the start_color and the bottom layer gets the end_color and the other layers get something in between. Note that
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Building the World's Greatest Cameraman Using After Effects 5.5 Expression Controls 
