Creating a bump map from hand painted material that uses multiple textures


Hi! :slight_smile:

I am making a missile/bullet for a railgun I am modeling. (here is the picture)

Now you might expect that I was normal set of diffuse, roughness, metallic maps. That is in fact not true but I am using diffuse, metallic and… I guess you could call it damage map? The “damage map” is like a gray scale image where a dark pixel means it is scratched and light one means not scratched.
I used this because I did not download any rust or beat up metal texture or anything like that. So with this approach I can with pretty much no work create accurate roughness and modify diffuse color. (white plastic spots and damaged/unpolished metal)
Here are the material nodes. (selected ones are immediately relevant)

How could I generate a bump map if the result after brightness & contrast and *-1 + 1 (aka invert) is heightmap?
If that is not possible how could I accurately imitate the brightness & contrast node? (how would I convert the 1.9 brightness and 40.7 contrast from blender into paint net, gimp or really any image editing program)