In Europe we have some rules that say that rectangular ducts have NO center
lines but round duct do have center lines.
So thats why we have to see the difference between round and rectangular
In AutoCAD MEP Autodesk made a API for doing this.
I Tryed to turn it on in the visibility graphics userinterface but I do not
see the result I want.
You can not make a difference in visibility for round and rectangular ducts
like in AutoCAD MEP.