Geometry problems

 From:  Michael Gibson
656.19 In reply to 656.14 
Hi Will,

> I use Unwrap3D a lot of times as a file translation tool and he grabs
> the NURBS mesh out of the 3dm and lets you work with it in the program...

Depending on what he reads, this may not work with 3dm files from MoI, because MoI does not store the display polygon mesh for the NURBS surface inside the .3dm file in order to keep the file size smaller.

So unless he reads the actual NURBS surfaces directly (which is unlikely), it won't work, since there isn't any kind of polygon data stored in MoI's 3dm files. You'll likely need to use a polygon format like obj for that still.

> I'm wondering if C4D does some special checks to look for flipped around normals
> and brings them back in line with the majority?

No, it's not that, I haven't yet seen a file where MoI put out incorrect normals interior to one single object that needed to be flipped.

It's a combination of 2 different things that it does well. First, it uses the vertex normal smoothing information that MoI writes to the OBJ file. Gent gave a great description and comparison of that above. This smoothing information that MoI writes comes directly from the NURBS model itself so it is very accurate. When a polygon program tries to calculate its own smoothing just by averaging perpendiculars from surrounding facets, it doesn't work very well unless the polygons are all pretty evenly sized and distributed..

Then the second thing is that it can triangulate a complex concave n-gon correctly without trying to put in triangles between vertices that make the triangles go outside of the n-gon boundary. In order to render pretty much everything has to eventually triangulate the n-gon and if it doesn't produce a good triangulation you will get a mess. This is typically the main problem that prevents good n-gon imports in many applications. It's unfortunately somewhat difficult to code a really robust triangulation algorithm.

Unfortunately there isn't any way in the OBJ file to store the triangulation of an n-gon in addition to the n-gon itself - it's up to the application that reads the OBJ file to calculate the triangulation properly...

- Michael