After I've joined 3 meshes into a single mesh with ctrl+j, part of it becomes transparent after I import it to Unity (see left side of the image). What could cause this and how can I fix it?

enter image description here

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    $\begingroup$ Looks like back-face culling to me, which would make sense if the normals of the bottom object were inverted. Try selecting everything in edit mode and pressing Ctrl N to recalculate the normals before exporting. $\endgroup$ – gandalf3 May 14 '15 at 20:24
  • $\begingroup$ Did you have any modifiers on the meshed before you joined them? $\endgroup$ – Denis May 14 '15 at 23:47
  • $\begingroup$ @gandalf3 It's strange, I had tried doing that earlier and it didn't work. Now after some more modifications to the model I rejoined the objects and found different parts to be transparent than before, and recalculating the normals fixed it this time. Thanks. $\endgroup$ – mattboy May 15 '15 at 12:26
  • $\begingroup$ @Denis Nope, no modifiers! $\endgroup$ – mattboy May 15 '15 at 12:26

It looks like the normals of the bottom part are inverted, resulting in a transparent look due to back-face culling.

Try recalculating the normals with ⎈ CtrlN in edit mode (↹ Tab) with everything selected (A). Note that it's possible to create topology which cannot have consistent normals, which could be why this didn't work for you before.

  • $\begingroup$ I've an object showing transparent faces (and black triangles), which I tried to fix after a Join. The object is not detected as non-manifold, normals are consistent and the problems are still present. $\endgroup$ – mins Dec 30 '19 at 16:05
  • $\begingroup$ @mins this is a wild guess, but is it possible you have a copy of your mesh overlapping in the same spot? $\endgroup$ – gandalf3 Dec 30 '19 at 20:11
  • $\begingroup$ Not the case. I found a bypass: Exporting the object (x3d) and reimporting it to create a clean mesh. $\endgroup$ – mins Dec 30 '19 at 23:40
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    $\begingroup$ @mins glad you got it working, happy new year :) $\endgroup$ – gandalf3 Jan 1 '20 at 12:28

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