Modding Help (solved) dungeons with worldgen-selected background but empty foreground?

I want to create a microdungeon where some tiles have air foreground and worldgen-selected background (so that it blends with the world, and looks as if someone just dug through it).



As far as I can tell, using TMX-style dungeons does not allow this, as it only provides:

1. Background AND/OR foreground Biome variant #1
2. Background AND/OR foreground Biome variant #2
3. Background AND/OR foreground Biome variant #3
4. Background AND foreground worldgen-selected (via leaving the tile empty in the foreground, and using magic pink for the background)

I know this is possible with PNG-based microdungeons, by using the following ".dungeon" json brush config:

Code:

{
  "value": [200, 200, 200, 255],
  "comment": "Clear foreground, leave background as-is",
  "brush": [["clear"], ["surfacebackground"]]
}

Apparently this doesn't work either.

 

I figured it out, the following brush will leave the background and clear the foreground, allowing microdungeons to seamlessly blend with the surrounding terrain.

Code:

{
  "brush": [["front", "metamaterial:empty"]]
}