Piano Visualizer

So did you ever get this to work?

Your workflow is a lot more complicated than it needs to be. If I were doing it I would find the layer that is generating the original piano roll, duplicate just that layer, name the copy Piano Roll 2, then put it in your main comp to drive the animation of the lighting effect you have generated. You should be able to get a comp with the Piano Roll 2 and the lighting effect to work just fine. 

Did you download the sample project? Does the Piano Roll layer have a transparent background? If it does not you need to change the [3] at the end of the alpha = so that it uses one of the colors. The sampleImage method returns an array of 4 values, RGBA or [0, 1, 2, 3] and you can only use one of them at a time in this expression.

Try this:

1. Start a new comp
2. Add a small rectangle shape layer with any kind of fill or stroke
3. Name the shape layer Piano Roll
4. Duplicate the Piano Roll layer and add a directional blur of about 40 pixels
5. Add small colored solid to the comp and name it test
6. Add the following expression to the small solid's opacity property

target = thisComp.layer("Piano Roll 2");
point = thisLayer.position;
alpha = target.sampleImage(point,[.5, .5])[3];
linear(alpha, 0, 1, 0, 100)​

• Parent the two shape layers together and turn off the visibility (eyeball in the timeline) of Piano Roll 2
• Drag the parent of the shape layer around so that it passes by the solid and the opacity should increase as you get closer to the solid
• Reverse the test and drag the "Test" layer around and watch what happens as it moves over the shape layers

As the anchor point of the solid "Test" layer approaches the blurred edge of the shape layer you should see the opacity increase. There is really not much to this. As long as the anchor point of your lighting effect ('keylight") layer is over the center of the effect any change to the alpha channel of the layer that is sampled (piano roll) will cause a change in the opacity of the layer with the expression.

One More Time: If your piano roll layer does not have a transparent background then you'll have to sample one of the color channels. You can pick any one of them. Red is [0], Green is [1], and Blue is [2].  That is the end value in the alpha variable in the expression. 

To get that custom lighting effect to work you'll have to get rid of the pre-comp put all of those layers in the main comp. If you then move the anchor point of each of those layers directly over the appropriate piano key you can add an expression to each layer to change the opacity based on the layer position. If you duplicate the piano roll layer, turn it into a guide layer, and add directional blur, you can ramp the opacity up and down instead of just turning it on and off. The comp would look something like this:

And here's the expression:

target = thisComp.layer("Piano Roll 2");
point = thisLayer.position;
alpha = target.sampleImage(point,[3.5, 3.5])[3];
linear(alpha, 0, 1, 0, 100)

 It's not going to render very fast, but it will work. 

You could make it work with all of the lighting effects on a single layer and modify the expression to adjust the values of each parameter by modifying the point variable and the tMin, tMax, Value1, and Value2 portion of the expression. It all depends on what you want to do and which parameters you want to animate.  You could even add a bunch of nulls to the comp and use the position property of the nulls as the point variable. There are a lot of ways to go but I think this is the easiest.

Here's a sample comp that is greatly simplified: https://www.dropbox.com/s/mxsr09uer2zf0js/piano%20roll.aep?dl=0

(if your browser adds a .txt extension when you download the file just delete it.)

I don't know how you set up your lighting effect layer. You mentioned Keylight but that effect is usually used to separate foreground elements on footage that has a consistent background color. To make a scenario using some kind of keying effect I would have to see the layer and know all of the modified properties and your workflow. 

If all you want to do to is generate some kind of glow every time one of the white bars intersects with the moving piano roll by far the easiest solution is to have the entire animated piano roll on a single layer, duplicate the layer, create a shape layer that covers just the intersection of the piano keys and the piano roll layer, use that as a track matte, then add a glow effect and/or some blur and color effects to the masked top copy of the piano roll layer. Something as simple as this:

If you want to use the "sampleImage" expression method to detect where the intersection takes place and control something like lens flare brightness then look at this solution presented by Dan Ebberts: Using Sample Image

You will end up with a lot more complex composition but a different look. Something like this:

The problem with this approach is that effect(index) does not seem to work as I would expect it to so you have to manually rename every instance of the lens flare, and every image sample takes time to calculate, so this approach would much slower to render.

Let me know exactly how you created your Keylight layer and how you expect to add the lighting effect to each layer and maybe I can help but I think your workflow may be a lot more complicated than it needs to be.

There are at least two recent threads on this forum discussing different techniques for this type of piano roll animation. Try doing a search for them. 

I would use an expression tied to the position of the light effects using the sampleImage(point, radius = [.5, .5], postEffect = true, t = time) method found in the Layer>General section of the Expression Language menu in the timeline. This will give you a color value in a 4 digit array from the position of the point. To start I would define "point" as being equal to the position of a null or an Effect>Expression Controls>Point. control that is attached to each key. 

You'll find solutions in the other posts. Sorry, I don't have time to look them up for you.