I was curious about this myself, so I asked our coder, Ben how it worked:
Basically locate outside edges on all the geometry passed as occlusion geometry (this is just a case of finding edge id’s that aren’t duplicated (by that I mean share the same verts in any order) then take each outside edge and build a stitching quad.
The stitching quad is just a zero sum face that has all of its vertices mapped exactly on the the outside edge. The trick is to make sure that one edge of the quad has normals that point out from the source mesh and one edge has normals that actually point out from the edge the stitch quad is generated from so they should be relatively perpendicular.
Then in the vertex shader, calculate the vector from the view position to the vertex and dot product that with the normal (vertex normal) both normalised of course. If the result is > zero add an offset to the current vertex position.
We actually use the flipped version of that, but the principle is the same.
Hopefully that helps! Let me know if you need more information. I’m hoping to cover this in more detail in an upcoming stream / youtube video as there’s been a fair amount of interest
