Why are non-manifold faces bad

cgphoenix

Ok I have been trying to find out why I should not use non-manifold faces.
I under stand that you should not have any non-manifold faces for 3D printing.
Is it ok to use non-manifold faces models in a game and what problems do non-manifold faces give I am finding it really hard to find info on this.
So I am grateful for any info you could give.
Sorry for the bad English.

FelixL

I believe the non-manifold faces like this shouldn't be a problem:

However, faces that can result in invalid triangulation are sometimes referred to as non-manifold as well.
For example the red polygon here:
It can be triangulated either like in the middle, which would be fine, or like in the right , which would result in a zero-area face. In most engines, the face tangent gets calculated by the face normal and the UV orientation, and if a face has an area of 0, you can't calculate a proper normal from it. This also explains why (at least in cryengine) any objects needs proper UVs to have clean shading, even if it's untextured.

marks

Perhaps I'm not seeing the same problems you guys are, but those look like things which can be reasonably worked around to me (in the first case by bending vertex normals, the second case being like ... just wtf are you doing, that's weird triangulating it like that).

FelixL

marks wrote: »

Perhaps I'm not seeing the same problems you guys are, but those look like things which can be reasonably worked around to me (in the first case by bending vertex normals, the second case being like ... just wtf are you doing, that's weird triangulating it like that).

Well, I'm just saying it can get triangulated like that without the user interfering, max's triangulation isn't fool-proof. Inside 3dsmax it's not a problem, but it becomes one once you export.

cgphoenix

Thank you guys! the comments was very useful