How did he do that ?

Unity Engine
1

I was wondering how to get started and code something similar (sorry I’m a complete n00b to shaders).

2

Literally the next tweet is the shader code he used. Though if you’ve never delved into Shaders how to use that probably isn’t very obvious.

The short version is you need to make a renamed copy of the TerrainSplatmapCommon.cginc and modify the SplatmapMix function to apply the function they posted to the splat_control. Then you’d need to make copies of the relevant terrain shaders (FirstPass.shader and AddPass.shader for example), point those at the copy of the TerrainSplatmapCommon.cginc file, and then at each other.

There are several resources for beginner shader devs on the forum, but this is one I like to point people at: