I’m trying to pull of an effect that used to be done in UE3. Using a custom lighting material you could essentially interpolate the normals of two intersecting meshes to eliminate the seam it creates with clipping polygons. This tutorial explains it perfectly: Damian Lazarski (search for the section called Blending props with the terrain). The problem is that UE4 doesn’t have custom lighting materials anymore. Someone on the Answerhub suggested manipulating the Light Vectors from Blueprints in place of custom lighting, but I’m unsure of how to go about doing that, or if that’s even the most appropriate way to handle this problem.
I think there is a better way of doing that same trick in UE4. I did it on fortnite once and it worked well.
By default materials have the “Tangent Space Normal map” option checked. That is what transforms the normal from tangent to worldspace. Uncheck it on your rock and landscape material. For the parts of your materials that do DO want transformed, do a World->Tangent vector transform.
If you want to reference the underlying normal from the terrain on an intersecting rock mesh, simply reference those material instructions (same as your link so far), and then don’t transform the “grass” normal from World->Tangent
Notice there is a (imo) huge problem with your example link. notice they removed all the vertex lighting directionality from the normal on the entire grass part of the landscape. That is no good, it will make the whole thing look flat.
The way you’d approach this with my suggestion is to use a paintable landscape layer to blend between the “Transformed” and “untransformed” grass. So you will need to paint “untransformed grass” normal on the landscape surrounding any intersecting meshes. What that will do is simulate a little flat spot which is considered “common ground” between the two materials. Then the rock mesh will get the “untransformed” grass painted on its edge which will be “flat” (aka only texture normal details not vertex normal will read) and in theory seamless (minus static lighting and shadowing issues which may crop up).
Or you could just do like the tutorial and have untransformed grass for the entire landscape.