Hi Chris,

> First I have to say that I don't have Moi yet, but I plan on getting it in the near future, so
> this question may come from my ignorance of Moi's export cabability.

I would definitely recommend giving the trial version a try, you can get a 30 day trial version from here:
http://moi3d.com/download.htm

With the trial version you'll be able to test the export yourself and see if it will be good for what you need or not.


> Would the fact that it is a 56Mb nurbs file present a challenge to making a mesh that is optimized.
> I could see that translating to an OBJ that is enormous, paticularly since OBJ has no binary format?

Yeah at 56Mb it's certainly something a lot more complex than just a model of a box. It will probably make a fairly large OBJ file, but a "large file" these days is not such a problem - we're not in an area of 1.44MB floppy disks anymore, something like a 300MB OBJ file is not usually a problem these days.

There is usually a pretty substantial increase in file size when converting a complex NURBS model into polygons, especially if you want it to look smooth since in order to make things look smooth with polygons you need to generate a whole bunch of little ones.


> Can Moi export an OBJ with quads?

It can export an OBJ with n-gons - it will be quads in internal areas of the surface and n-gons in areas where the surface has been trimmed - it basically starts out with quads on the base surface and then n-gons are formed where the surface has trimming information. This n-gon structure gives it a clean wireframe look but it is not the same as "all quads" output.

MoI is generally focused on producing an output mesh that will be used for rendering - if you're worrying about all quads that might mean that you're thinking of applying sub-d smoothing on to the mesh? If so then no - MoI's output is not intended to be used for sub-d smoothing, that really requires hand tuned topology and you will usually need to create a sub-d cage by hand in a sub-d modeling program and not as some automatic generated output. You can use the MoI output as a starting point for a retopology process though.

See here for some previous discussion:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4923.1


But if you want to generate a rendering, there is no need to have "all quads" for that anyway.


When you export a mesh from MoI (or other CAD programs too), it also exports vertex normals into the OBJ file and those vertex normals come from the original NURBS surface data from the CAD model, so things like all the polygons that make up a sphere piece have exact vertex normals from a sphere. So when those polygons are rendered they get shaded with those original surface normals and that makes the rendering look exactly the same as the original model.

So because of this you usually don't want to mess around with trying to additionally "smooth" the output, instead of making it smoother you would actually be deforming it and losing the accurate vertex normals in the process, making things worse instead of making them better. When you export out from MoI, all the smoothing information is basically already in the OBJ file in the vertex normals, you just render that directly and you get perfectly smooth looking shading. You really want to preserve that vertex normal information and not do anything that would cause it to get discarded.

Usually CAD models are in their precise shape already, with smooth surfaces in areas that are supposed to be smooth - so typically it is kind of a weird idea that you would want to "further smooth" the shape of the model - normally with CAD models that would disturb the precise shape of the model, things that were currently exactly spherical would get kind of melted to a somewhat different shape. So that kind of thing is not normally part of the process of what you do with models that you export from MoI.


It does not seem to be that unusual for someone who is coming from a sub-d modeling background to think that they must always apply sub-d smoothing to everything they're doing - but when you're rendering CAD data it's a different situation and you do not want to apply sub-d smoothing in that case, you just load the model and render it. Usually you don't go in and squish any individual points around since that will invalidate the vertex normals that are going to give the precise smooth shading to the rendering.


- Michael