Hi Lewis - the easiest way to avoid worrying about flipped normals is to just use a double sided render material like you mentioned. Or another way that works easily is if you form your object into a completely closed solid volume, that will have normals oriented towards the outside of the volume.
If you have a surface or set of joined surfaces that does not make a solid volume, it is ambiguous which side of it is considered the positive surface normal. There is not currently any tool in MoI set up to visualize which side is the positive side, mostly because there isn't really anything in MoI itself that is sensitive to that. I had assumed earlier that if a program was sensitive to it, that they would also include tools to flip it if needed but yes unfortuantely often times there are bugs with just simple flipping where they don't flip vertex normals as well. I would think you should report that as a bug and see if they will fix it, it should not be a difficult thing to flip vertex normals at the same time as flipping polygons.
Although there isn't a way to visualize in Moi which side is the positive normal side, there is a Flip command that you can use to flip an open surface to the opposite direction:
http://moi3d.com/3.0/docs/moi_command_reference10.htm#flip
So if you do know which ones need flipping use the Flip command on them.
The other thing which can help a lot is if you make sure to join surfaces that are adjacent to one another even if you can't get them into a closed solid. That's because joined surfaces will at least have a consistent normal direction across the whole structure and it will be easier to deal with larger chunks of surfaces that act as one rather than a whole bunch of totally separate individual surfaces.
In the future I do want to add an option for displaying one side only or probably drawing backfaces in a fixed color like red but currently there isn't anything for that, sorry.
- Michael
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