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The 'Pan Behind' Tool – Adding an expression to the background

Adding an expression to the background
That’s the basic setup. But I’m going to be panning the foreground layer while leaving the “magnified zone” (the mask) in place. So I’ll also want to pan the background. To save myself a little headache, I’ll use a simple Expression to make the background pan along with the foreground. (This will also help me visualize the final effect interactively as I manually keyframe the foreground.) For those of you inexperienced with Expressions (and, no, I’m no expert myself), here’s how that process works. (Note that this step is not integral to the pan-behind effect; it’s simply here to show you how I did it in my sample project.)

 1. In the timeline, expand the Transform” parameters for the two newspaper layers so that both Position parameters are visible.

2. Option-click (Macintosh) or Alt-click (WIndows) on the stop watch next to the background layer’s Position parameter. This adds a blank Expression to this parameter.

3. You’ll also see a little “Pick Whip” button appear.

Click and drag this onto the Position parameter of the foreground layer.

4. Then you’ll see this expression (or something like this) show up in your timeline.

Select the text in the Expression so that it reads, “thisComp.layer(“newspaperforeground”).position/2+150.”

That essentially sets the position of the bottom layer to follow the position of the foreground layer at half the rate and offset by 150 pixels, which happens to work for my composition. You might change the 150 to a higher or lower number, depending on the dimensions of your composition. (Again, if you need the help, you can download my sample project by clicking here.)
Finally, if you need to make any adjustments to the position of the background to get it lined up properly to begin with, you can do this by changing the location of the anchor point on the background newspaper layer. This will allow you to set the base position of the background layer manually before it begins following the top layer around, as you’ll see below.