Material that only shows shadows cast upon it

Development Rendering
1

Hi everyone,

I’m making an isometric game with pre-rendered backgrounds, but the characters and some of the assets will be 3D. I’m using the geometry for the backgrounds for collisions, but I want to apply a material to it that renders it invisible except for shadows cast upon it. I want to give the illusion that the shadows are cast onto the background images.

Here’s an example of one of the scenes showing a 3D character superimposed on a 2D background:

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Also wondering if there’s a way to occlude 3D geometry with a material.

Any help would be greatly appreciated!