How to use "the distance and angle between the object and the protagonist" to change the "strength of the object's material" or "change the material's PNG image"?

Godot Version

4.2.1

Question

Title = How to use “the distance and angle between the object and the protagonist” to change the “strength of the object’s material” or “change the material’s PNG image”?

I chose “Billboard”, but it’s not all facing the camera.
I want to have a piece of paper facing the camera completely,
If the world direction of the “3D camera” = the world direction of the person, and the height position is directly behind the person,
Then the picture on the paper will be changed to “Backside of person.png”. If it is the other way around, change it to “Front of person.png”.

If the “object” in the 3D world is farther away from the “protagonist”, the “black intensity” of the “object” is stronger.

If the “person” touches the “shadow 3D range”, the closer the “person” is to the “origin of the shadow 3D range”, the stronger the “black intensity” of the “person” will be.

You can setup variables that shaders can react to. Then have your script detect the stuff you require and set those variables. The rest happens automagically.

Will go look at docs quickly.