Jeff Lait

This is hard to answer as the goal could be many things...

Assuming you have some hard-ish cut-out that you want to soften.

Often this means there is no "data" in the cut-out parts - so things like blur that expand the softness outwards can't work, as there is no idea what the colours should be there.

This often thus requires a pure inward feather. This is what feather can do, the trick is you want to do it only on Alpha.

So, rgba -> rgb/a split. Then feather on a. Then rgb/a -> rgba to combine and re-multiply. If you have a zero-one alpha, you can use a remap to adjust the shape of the fall off to make it less "linear".

Attached shows this in action.

Rastering UDIMs with adjacency won't help you much, as when you go to _use_ the adjacency maps you then need to jump between all possible UDIMs when you leave an island.

You need to make a utility-texture coordinate system for use with the adjacency maps. It often is useful to have a separate texture coordinate for adjacency as you can increase padding. Then the idea is to synthesize the adjacency aware operations through this utility map. Then each UDIM can read off the utility map to get its values.

I don't think you missed any obvious existing solution :<