How do I use a subtractive geometry brush?
You should be able to use the information in this page to help you use BSP additive and subtractive: https://docs.unrealengine.com/latest/INT/Engine/Actors/Brushes/index.html
In essence, all you need to do use BSP additive/Subtractive is use them set the drop down for which method you need. As a note, the subtractive BSP volume will only work with other BSP brushes and will not work with static meshes.
I hope this helps.
answered Jan 27 '16 at 02:14 PM
Tim Hobson ♦♦ STAFF
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