# Extruding hollow cube making interior disconnections **URL:** \ **Category:** C++\ **Tags:** faces, unreal-engine, bsp, geometry, question\ **Created:** [January 7, 2018, 6:22pm UTC](https://forums.unrealengine.com/t/extruding-hollow-cube-making-interior-disconnections/413996 "2018-01-07T18:22:46Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![anonymous\_user\_7897ae0a](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@anonymous\_user\_7897ae0a](https://forums.unrealengine.com/u/anonymous_user_7897ae0a)\ **Post date:** [January 7, 2018, 6:22pm UTC](https://forums.unrealengine.com/t/extruding-hollow-cube-making-interior-disconnections/413996/1 "2018-01-07T18:22:46Z")
Hello! I am learning about Geometries and trying to extrude a hollow cube. It looks fine on the outside, but inside, the old faces extruded from are like walls separating the original space from the extruded space, and the extruded space is invisible from the inside. Fine, I delete the old face-walls and start creating faces inside the extruded area, but they appear to be detached from the original area. The interior shot shows a gap between the original left and the extruded right. ![226284-interior.jpg](https://d3kjluh73b9h9o.cloudfront.net/original/4X/3/4/a/34a5a7dab1834a95b23cf2c2212787747f0a1555.jpeg) However, the exterior shot is continuous, what I hoped the inside would be. ![226285-exterior.jpg](https://d3kjluh73b9h9o.cloudfront.net/original/4X/c/4/e/c4e3ee2edd66d10ea963f1d9696c1c1bc24cb1aa.jpeg) Here’s a view of the exterior when selected, the original on the right and extruded on the left, in case that is needed. ![226286-highlights.jpg](https://d3kjluh73b9h9o.cloudfront.net/original/4X/1/2/8/128387da433a1790c7a7d2babba87c3ae78500b4.jpeg) So, why is the extruded area separated from the original area? How do I put them together into one continuous interior area? Thank you for your time. ---
**Author:** ![ThompsonN13](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@ThompsonN13](https://forums.unrealengine.com/u/ThompsonN13)\ **Post date:** [January 8, 2018, 1:33am UTC](https://forums.unrealengine.com/t/extruding-hollow-cube-making-interior-disconnections/413996/2 "2018-01-08T01:33:17Z")
The reason it appeared invisible from the inside is due to the normals of the geometry. as for the first picture it looks like a gap between two separate pieces of geometry. ---
**Author:** ![anonymous\_user\_7897ae0a](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@anonymous\_user\_7897ae0a](https://forums.unrealengine.com/u/anonymous_user_7897ae0a)\ **Post date:** [January 8, 2018, 6:44am UTC](https://forums.unrealengine.com/t/extruding-hollow-cube-making-interior-disconnections/413996/3 "2018-01-08T06:44:41Z")
I see what you mean. Why would that be if it’s the same BSP with the piece extruded? ---
**Author:** ![Solutions](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Solutions](https://forums.unrealengine.com/u/Solutions)\ **Post date:** [January 8, 2018, 7:11am UTC](https://forums.unrealengine.com/t/extruding-hollow-cube-making-interior-disconnections/413996/4 "2018-01-08T07:11:25Z")
That would be because a polygon is created between the vertexes you extrude from. ---
**Author:** ![anonymous\_user\_7897ae0a](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@anonymous\_user\_7897ae0a](https://forums.unrealengine.com/u/anonymous_user_7897ae0a)\ **Post date:** [January 8, 2018, 7:32pm UTC](https://forums.unrealengine.com/t/extruding-hollow-cube-making-interior-disconnections/413996/5 "2018-01-08T19:32:09Z")
Think I figured it out! Part of setting a BSP to Hollow is that you can also set Wall Thickness. On a whim, I tried setting that to 0, and no gap. This shows that the gap was really the “wall”. I’d still like to know why extruding a hollow puts walls between the segments though. I would delete the question because it seems dumb to me in retrospect, but I’m sure other newbies like me out there could use this. ---
**Author:** ![anonymous\_user\_7897ae0a](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@anonymous\_user\_7897ae0a](https://forums.unrealengine.com/u/anonymous_user_7897ae0a)\ **Post date:** [January 8, 2018, 7:32pm UTC](https://forums.unrealengine.com/t/extruding-hollow-cube-making-interior-disconnections/413996/6 "2018-01-08T19:32:57Z")
I figured out what that polygon is: the wall, which had a greater thickness than 0. Thank you guys for taking the time to speak to me about it.