Hi, I want to force the flow of polygons and edges of my exported topology on an fbx. Is that possible?

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.

Any tips?

Thanks

I received a reply to this from Michael. I am posting it here for others:

Hi Simon – mesh topology is inherited from the UV grid structure of the “underlying surfaces”, which are the base surfaces used in a NURBS model. The way NURBS modeling works at a fundamental level is that a surface is made up of a regular row/column grid of control points which defines a UV layout to it, and then there can be trim curves that mark various zones of that surface as either an outer trim boundary or as an interior hole.

When MoI goes to make a mesh from a surface, the meshing process first generates quads along the UV directions of a surface, and then those become n-gons where the quads hit trim boundaries.

When you split an object by doing a boolean or a trim operation, it’s a fundamental feature of NURBS modeling that those “underlying surfaces” remain exactly the same as before and it’s only new trim curves that are created on the surface. That’s why Booleans work so much better in NURBS modelers than in polygon modelers, because boolean operations don’t cause things to fragment into a zillion little pieces for every individual cut.

But because of how that structure works it is normal that you will not see changes in topology just by splitting an object.

Really the best way to influence topology is to process the output mesh with retopology tools in a polygon modeling program. That’s the standard way to achieve any particular desired polygon topology and you should not generally expect to generate a hand tuned topology from an automatic generated mesh.

It is possible though to influence the topology in MoI by reconstructing specific surfaces by extracting edge curves by selecting the edge and running copy/paste on it, and then delete your trimmed surface and construct a new surface using something like sweep or loft which will generate an untrimmed surface as its output which will then mesh with a topology that follows that particular new construction.

Like in your example there delete the strip piece, and then select the 2 edges that are now open, do a loft between those to build a new simple untrimmed surface and then use Edit > Join to glue that new surface into the original structure. Now the exported mesh will be different in that area, the edges of the surface will be natural edges of the underlying surface rather than trim curves on some larger extended underlying surface.

But really it’s best if you need specific topology to plan to do retopology in your polygon modeling toolset.

Hope this helps give you some tips though,

Now that I read your question again it looks like I didn’t quite answer you directly, you asked:

“In the picture below I have had to split the object into separate parts to achieve this but that means I would have faces in unwanted places when I export.”

There is not any way to have mesh generation influenced just by some curves on a surface, and it’s not really influenced heavily by splitting either since as explained above that just introduces new trim curves but keeps the same underlying surfaces. If you want to influence the mesh generation you would need to not only split objects up but also reconstruct key faces as untrimmed surfaces by building new ones using loft or sweep tools and using the curves from the split as part of the inputs for the loft or sweep.

If you need to influence the mesh topology to have particular edge loops, you should expect to that during a retopology stage in your polygon modeling program. That’s not something that is going to be controllable with the mesh export out from MoI.

The mesh export out from MoI should be suitable for rendering directly without needing to have any specific topology to it though, you generally only need specific types of topology if you want to apply sub-d smoothing to an object but usually with NURBS models they are already in their final form with smoothing already applied as fillets and so it typically does not really make sense to try and further smooth a NURBS CAD model…

- Michael

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