Hi Simon, I just sent you a reply through e-mail with a lot of description in it.

To answer your question here though - no it is not possible to alter the exported polygon mesh topology just by the supplying some individual curves on a surface.

In order to adjust the topology the main way you would need to do that is to take the exported mesh as a starting point and then use retopology tools in a polygon modeling program to draw out the specific topology that you need.

One other option though would be to reconstruct certain zones of your object by deleting faces and building new ones in using surfacing tools like lofting or sweeping between those edges. The difference is that if you just split or cut an object using booleans or trim tools that does not alter the underlying surfaces, the base surfaces stay the same and only new trim curves are created. Since the base polygon topology follows the UV quad structure of the underlying surfaces, just splitting an object does not have a big effect on the generated topology, the quad flow still just follows along the same underlying surface as before there are just new n-gons formed where the new trim boundaries are at.

So you would need to do surface reconstruction building new base surfaces in order to have a big effect on the generated topology.

You would not normally need to be concerned so much about the topology for the regular use of just rendering mesh output though, you only really need to have a quad edge flow topology if you plan on doing sub-d smoothing to the mesh output, but that is not typically what you do to CAD mesh output since usually CAD files are already smoothed with things like fillets in places that are supposed to be smooth. You can just render the direct output from MoI and it will look smooth because of the vertex normals that are used in the render. It generally does not make sense to try and apply further sub-d smoothing to a CAD model because that would mutate things away from the current precise already smoothed model shape.

If you are looking to generate sub-d friendly mesh topology though then the standard way you should plan for getting that is to use retopology tools in your polygon mesh editing program though.

- Michael

Hi Simon, and just to clarify:

> I thought that in some way I could create new edges inside the faces of an object (such as a circle
> on the side of a cube) but I can't seem to do this.

Just drawing a circle on the side of a cube does not in itself introduce new edges - you just have a separate circle that happens to be located there. (Some day I may try to make some kind of drawing mode where this could be different though and have drawn curves auto trim things but that does not happen currently)

In order to introduce a new edge you can use the Edit > Trim command to actually cut the surface of the cube by that curve and then you'll have edges there after that.

But again that will have a somewhat limited effect on the topology of the generated mesh because the underlying UV quads will still be the same as before, you will just get new n-gon structures where the quads run into those newly introduced surface trim edges.

In order to have a larger effect on the topology you would need to construct the specific surface using a tool like loft or sweep that generates a simple "untrimmed" surface to start with where the surface's UV flow is matching that specific curve, rather than the curve being a trim curve that is just marking a zone of the surface as a cut away area rather than directly defining the UV structure of the surface itself.

This all comes from the concept in NURBS modeling where you can have "underlying surfaces", where surfaces can be big broad sheets that then have trim curves that live on top of them and mark areas of the surface as the outer boundary or as interior holes.

When you do a boolean or trim operation, all the "underlying surfaces" remain the same as before and only new trim curves are calculated. This is fundamentally why booleans work much better with a NURBS modeler than they do with a polygon modeler though, because surfaces remain simple and accurate underneath things as you do more booleans and cuts in different areas, rather than things fragmenting up into tons of little tinier and tinier bits like happens with polygon booleans.

- Michael