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Whenever two faces on two intersecting models are co-planer, booleans completely break.

Steps to reproduce:

  1. Create a new file with the default cube.
  2. Select and duplicate the default cube and offset slightly in the x, y, or z axis only.
  3. Change the draw mode of cube.001 to wire only.
  4. Add a boolean modifier to cube and have it point to cube.001
  5. Error:
    1. If the mode is set to intersection, the result should be the narrow rectangular region between the cubes. It's not.
    2. If modifier is set to union it should yield the exterior of both cubes stuck together. It doesn't.
    3. If the modifier is set to difference it should yield only the overlapping region. It doesn't.

As soon as you move one of the cubes so the faces are not co-planer anymore, the modifier behaves as expected. Carve seems to work better than bmesh but neither works properly. How can this be overcome and why is this happening?

amoose136
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    "How can this be overcome and why is this happening??" > "two faces on two intersecting models are co-planer" make them non-coplanar or non intersecting, seems to me like you already know the answer. If the faces are intersecting Blender can't figure out "in" from "out" – Duarte Farrajota Ramos Jul 13 '17 at 19:59
  • Please read through the following link: https://blender.stackexchange.com/questions/34781/boolean-modifier-not-working –  Jul 13 '17 at 23:15
  • @duarte They really must be co-planer or I'm making a shape I don't want with tons of extra geometry at the seam it doesn't need. Why can't blender know what's in and out when the faces are co-planer? Does it not use normals? – amoose136 Jul 14 '17 at 01:29
  • @cegaton this isn't caused by non manifold geometry, incorrect normals, doubles, or anything like that. We are literally just working with two cubes here. There are no solutions or insight to this problem at that link. – amoose136 Jul 14 '17 at 01:32
  • https://developer.blender.org/T37659 – bertmoog Jul 14 '17 at 05:20
  • I saw that but that's not the same bug. The vertices don't overlap in the scenario I've described. – amoose136 Jul 14 '17 at 07:52

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