How do make a layer visible only when overlapping other layers in After Effects

Question

How do make a layer visible only when overlapping other layers in After Effects.

Below are the layers:

I want the shadow from the bow to only be visible when over the verticle lines.

Below are the layers in the pre-comp:

Above is the semi-transparent stroke as the shadow if the bow and a repeated verticle stoke. I want that shadow layer to only be visible over the verticle lines. How can I achieve this?

Answer 1

You can use a Track Matte. If your vertical lines aren't already precomposed, precompose them. Duplicate the Precomposition in your main composition. Place the shadow layer between the two precomps. Set the shadow layer's TrkMat setting to Alpha. (If you can't see the Track Matte [TrkMat] dropdown press the Toggle Switches/Modes button at the bottom of your screen).