I have stumbled upon the following problem:
1) I import into blender an .stl file exported from the AutoCAD (even relatively simple model - something like cube with rounded corners i.e. dice)
2) I try to apply a Boolean modifier on it and an another mesh (created directly in blender), for example a cylinder, then I test every possible operation, i.e. dice union cylinder, dice intersect cylinder and dice minus cylinder.
3) the resulting object seems to be a result of the operation OPPOSED to the selected, that is:
3a) if I choose "union" then the result looks like "difference" (dice with part of cylinder removed)
3b) if I choose "difference" then the result looks like "union" (dice and cylinder together)
3c) if I choose "intersection" then the result looks like cylinder minus dice (sic!) (cylinder with the part of the dice removed).
It looks like the blender treats the dice as being everywhere EXCEPT the dice itself (a set complement to the dice). It is strange, because the normals seems normal ;) i.e. they point outwards from the dice surface (in N-menu I choose to show normals). Have anybody seen such a issue? Is there a way to "fix" this? Anybody knows what is the purpose of this?