Question Where do I start? Making a cuttoff shader.

I am wanting to have a shader where the brush's Opacity has a different output depending on highs/lows.

In the video, a higher Opacity Brush will yield the result on the left and a lower on the right.

I know the Blend Node is needed (Obviously) but I do not know where to go from there...

Any questions/clarifications, PLEASE ask.

I dont know how to use the Shaders, every tutorial I have watched does it for an entirely different context (Height maps etc)

 

So what exactly is your question? Do you want to make a painting system to paint terrain with variable density? Or you want to make the paint-brush shading effect (that cursor gradient)? Please clarify, but to me it seems you are talking about terrain painting.

 

Correct, I am talking about painting textures onto terrain.
Specifically the blending/cutoff of the edges.
In the video is what I want to achieve, but with the left terrain having a high Opacity and the right having a low Opacity.
I understand this is achievable with Shaders.

Question: How do I influence the blending via Shaders?