Hi Dave, just a note on this part:

> I say "strange" because selecting the bad ngon and flipping
> it did not fix the problem. But if you flipped it a second time,
> then everything was OK.

Yeah, getting that result is due to one of the bugs in Modo's OBJ importer.

In Modo's core functions, it is sensitive to which vertex of a polygon is set to the first one in the list of vertices.

Modo's core polygon normal calculator only looks at the edges radiating out from the very first vertex of the polygon in order to determine the polygon's normal. So with a complex n-gon, the first vertex must be set to a convex vertex in order to get the proper result.

As far as I can tell, Modo's OBJ importer fails to do that, which may result in an improperly oriented polygon in Modo.

When MoI exports to LWO format, it does this work to set the starting point of the n-gon to a good location for Modo/LightWave's orientation mechanism. So that's why this problem is avoided when using LWO export instead currently.


Since Modo has this sensitivity to which point is the first point, it would be a good idea for their OBJ importer to be aware of that and set the first point to match what their core engine expects...

But also another good improvement would be for them to tune up the core normal calculation so that it is not so sensitive to which point happens to be the first one in the list.

The reason why doing the flip twice works, is that there is a mechanism in Modo's flip function where if you do it the second time it will do some work to figure out a good start vertex for the ngon and rotate the points in the list so that one is first.

Some other discussion on this here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2039.5

- Michael

Hi Dave,

You should be lucky with the .obj in modo, because I've got thousands of n-gones problems when using this format.
As Michael said, .lwo is much better, I can't remember when I had a flipped poly using it ... but still it may happends.

So I'm still doing an geometry>poly>align when importing .lwo object ... it works as a double flip you descibe, but it's an 'autmaic' way
so it will resolve normal problem on area you don't always notice at first sight. (Can be frustrating to discover a wrong poly after an hires rendering of 4 hours for exemple).

Hi PaQ,

> As Michael said, .lwo is much better, I can't remember when
> I had a flipped poly using it ... but still it may happends.

Right now I don't have any reports of improper flipped polygons when using LWO format...

So I'd think that extra geometry>poly>align step should not be needed if you are using LWO format, but by all means if it makes you feel better... ;)


There are a couple of issues that can come up which are not actually bugs though, like if you export an open surface rather than a closed solid, it may not have the normal orientation like you expect because only solids have an automatic "outside" direction applied to them.

Also if you have 2 independent surfaces in MoI that are sitting side by side and not actually joined to have a common edge, then the meshes are generated separately and may not have the same normal orientation between them. When objects have joined edges, the polygons will have a consistent direction between them though.


What I have seen on a couple of rare occasions is a triangulation failure where Modo did not handle splitting up a certain n-gon into display triangles properly. This is a different problem than a normal orientation one, but it is also very unusual to run into it.

- Michael

In general, exporting from MoI is going to be fine for stills. However, if you expect to animate and deform your meshes you will have to get a good topology tool. Bang for the buck going MoI + SILO, or MoI+3DCoat would allow you to get your NURBS models into a program that you can tailor the polygons by retopologizing.

Hi Michael, ... well indeed maybe I have developed some OCD about poly normals :o)

I'm not quite sure what kind of mesh I will build in MOI if I know I have to do some deformation/animation with ... until ... I tried to export a model in ngone + divide larger than option ... the result is quite cool Oo

the original model was less funny =O)

Hi Jason, yeah like PaQ shows you can use the "Divide larger than" function in MoI's exporter to ensure that large polygons are diced up into smaller evenly sized pieces.

That can make a result that should be suitable for non-rigid-body type animations without necessarily needing to retopologize.

Certainly if you have a very specific kind of animation where you want one individual piece to move in a certain way it could be a good idea to retopologize that area though...

- Michael

Good point, good to try first without doing topo again.

What I like though is that MoI is getting to be an artists tool, so keeping in mind that MoI can play nice with modo, 3DC, SILO is a good thing.

> Hi Dave, just a note on this part...

Thanks for the detailed explanation, Michael!

I'm definitely switching over to using .lwo format from now on...

Dave Morrill

> So I'm still doing an geometry>poly>align when importing .lwo object ... it works as a double flip you descibe, but it's an 'autmaic' way
so it will resolve normal problem on area you don't always notice at first sight. (Can be frustrating to discover a wrong poly after an hires rendering of 4 hours for exemple).

Thanks for that tip, PaQ! I think I had tried 'align' on a Bonzai 3D model when I was evaluating it (they tend to produce lots of flipped polys all over the mesh), but it didn't seem to have any effect on the flipped polys. It's good to know that it fixes the problems if and when they occur in a MoI import.

Is there any trick to using 'align'? I think when I tried it on the B3D mesh I did not have any geometry selected. That is, since nothing was selected, I assumed it would work on the entire mesh in this case. Is that the right way to use it, or should I have something selected before using it?

Dave Morrill