http://archicad-talk.graphisoft.com/viewtopic.php?t=19924
The bottomline is to remove the "blank background" or "white fill" of ex-archicad drawings.
I use the Model View tweaking (for DWG convertion) to use the setting for all View Maps (pict below), by making a new name ("Store as...").
There is also another simple way:
right-click the view map drawing>
Section Settings.../Elevation Settings... >
Model Display >
UNCUT ELEMENTS >
Fill Uncut Surfaces with: >
" N o t h i n g "
It works as same as the first one, but only for particular setting edited View Map(s).
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AC11-1200