Hi sneather - is the object a closed solid or is it an open surface with some unjoined edges in it? Or is it possibly a big bunch of individual surfaces that are not joined to each other at all?

A closed solid should automatically get its normals oriented towards the outside of the volume, but if there are any anomalies in the object like messed up trimming boundaries or self intersecting surfaces those can interfere with the automatic orientation.

If it's not a closed solid then there isn't any "outside" that's well defined for the object and so you can easily have it in either direction. If you have a bunch of separate individual surfaces that are not joined to each other at all it is good to get them joined up even if it's not a closed solid because the joined surfaces will all have one consistent direction between them instead of each individual surface possibly being flipped from its neighbor.

If you know that you have an open surface that you'd like to flip it is possible to flip it in MoI using the Flip command:
http://moi3d.com/3.0/docs/moi_command_reference10.htm#flip

But typically the easiest way to deal with reversed normals is to just set double sided rendering to be turned on in your renderer, that way you don't have to worry about which side happens to be marked as the positive side, both sides just get rendered fine with that setting. So I'd usually recommend doing it that way since that solves the problem without having to do anything to the objects at all.

There isn't any way built in to MoI to see the normals direction while still in MoI though - I had assumed early on that any program that was sensitive to the positive normal direction would likely have a command in it to flip it, but unfortunately many programs are not able to do that properly because they forget to flip the vertex normals too even though it should be a simple thing to do. You might want to contact Maxon about that to ask them to fix up their flip command so that it also flips vertex normals as well, there isn't any reason that I can think of to flip polygon normals and not the vertex normals as well.

Hope this helps!

- Michael

Hi sneather, also another possibility is using the "Vertex Normal Tool" plugin for Cinema4D from here (scroll down to the second item on the page) :
http://frostsoft.blogspot.com/p/plugins.html

That plugin looks like it has a way to flip vertex normals in it, maybe that could be used to fix the vertex normals after you've used the regular flip command in C4D.

- Michael

Thanks, Michael. Yeah, the model is a little less than perfect. There are, indeed, some naked edges, as whomever was responsible for this product, didn't seem to model as well as they could have. However, not so many that I thought it would be a problem, but clearly, I must have underestimated that impact.

I wish I could just solve the problem in C4D, but the real issue, is that I'm using normal maps in my materials, and that immediately reacts to the opposing normal and vertex maps. And, yes, I've pestered Maxon for years about this flaw, and I've never even received as much as a response. They seem to focus on new features which look good on a sales page, but often neglect core details which make our lives hard.