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It seems that when I use the boolean modifier it switches the resulting boolean option between difference and union. This can't be normal right?

I'm using blnder 2.75 enter image description here

enter image description here

Delagone
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  • Can you explain better? You ask to difference the 2 meshes in figure one and nothing happens? – Fuboski Mar 16 '16 at 14:35
  • Yes, I ask to difference but I get an output that is union. and vice versa. – Delagone Mar 16 '16 at 14:36
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    Recalculate normals (Edit mode > select all > Ctrl+N). Related - http://blender.stackexchange.com/questions/46399/boolean-does-not-slice-correctly-using-difference – Mr Zak Mar 16 '16 at 14:50
  • It happens to me too. When I get the wrong boolean modifier and I can see it I just keep changing it until it works. – Sanix25 Mar 16 '16 at 17:32

2 Answers2

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This can't be normal right?

Funny, it is because of normal(s). Recalculate surface normals with Ctrl+N in editmode. Also make sure the scale of the object is not negative (apply scale Ctrl+A)

Normals specify what is considered outside and what is considered inside of mesh. Booleans take this into account.

Jaroslav Jerryno Novotny
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  • Would be useful if you mentioned how you can tell that normals need to be recalculated. – Ray Mairlot Mar 16 '16 at 15:18
  • Thanks! I feel kinda ashamed I dind't think of that. It didn't even occurred to me because I started out using basic shapes. – Delagone Mar 16 '16 at 16:26
  • @RayMairlot You can tell it needed this by the dark purple/grey color of the mesh geometry. – Rick Riggs Mar 16 '16 at 17:01
  • @RayMairlot There is no specific way to tell - when you scale an object to negative values you won't see a shading difference. There are other situations when shading looks the same. You can tell by the booleans working weird - that's about it. – Jaroslav Jerryno Novotny Mar 16 '16 at 17:20
  • @Jerryno I was alluding to the fact that the shading of the objects in the screenshot were dark, indicating inverted normals. I just thought it might be useful to include that that can help deduce that normals are the issue. – Ray Mairlot Mar 16 '16 at 18:05
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Let's simplify the example.

If you want to difference one mesh from another one like the example

enter image description here

Take 2 DIFFERENT MESH and in the first one select the modifier Boolean.

NB BEFORE DOING THAT REMEMBER TO CHECK (could be your problem) that both the meshes have faces not oriented inside. I.E Select first mesh - EDIT MODE - Ctrl+N and then you can use the modfier right

press apply and you did it!

enter image description here

Fuboski
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