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Control layer's opacity through another layer's alpha channel

Wed, 11/04/2009 - 20:10 — ssmith
Special thanks to: 
Dan Ebberts' "Obscuration using SampleImage()"

This expression uses a control layer's alpha channel to control the opacity in another layer.

EXPRESSIONS
Expression code: 

target = thisComp.layer("control");
alpha = target.sampleImage(transform.position, [.5, .5], true, time)[3];
linear (alpha,0,1,0,100)

A good way to test it is to apply the "checkerboard" effect to a layer called "control" and then paste the expression into the second layer's opacity parameter. Manually move the target layer around the comp and it's opacity will switch between 0% and 100% depending on the control layer's alpha channel value at those co-ordinates. Of course you don't have to reference the "control" layer's alpha channel, it can just as easily be one of the other channels.