Heyo,
I came up with a method for combining normal maps using a very simple node setup, using vector rotation. I haven’t seen it shared anywhere so decided to share it here.
This is the node setup:
You can combine two or more maps with this method without losing any detail. And you can control the strength of each map independently as a kind of mixing “Fac”.
One caveat is that it is technically possible to rotate the normals below the surface, however from my experience you need quite a few very strong maps for this to happen. You could solve this by implementing a maximum rotation on the final vector, but I haven’t included that here for simplicity.
I came up with this for my addon PBR Painter, which uses a similar (albeit slightly more sophisticated) setup to combine normal maps between texture layers.
I go through the theory in this video, although I am using an old approach whereby I do the rotation on the normal maps prior to the normal map node, which is a bit more cumbersome.
Comments/critiques welcomed! 