In the case where I don't render the ground, the position of the ground is black, can TileBuilder output a picture where the blank space is transparent?
This should be transparent as far as I'm aware of... can you send me your script file, so I can have a look? (info@stevencolling.com)
Edit: this was a bug and it is now fixed in the version 1.3 of Tile Builder! Thanks for pointing out the issue. :-)