This is haunting my brain for about a year now. I know its not that easy to make nurbs out of sds meshes
but I have this thought for so long that ... WHAT IF .... it would be possible to import poly meshes as an obj. and they would
stay reference objects like its already possible and we could have a plane this would also be a poly object thats native to moi 3d
and we could move that plane accross our poly model and make curves where the poly mesh intersects the plane.......
We would then have these curves to build up our nurbs surfaces... like using loft or network..... (the plane could intersect in all 3 dimensions if we do that manualy)
Michael any idea if that could be made.It would be awsome for both worlds poly and nurbs
Peter

Heres the 3 dimentional manual scanning..... you would just move the plane and make a curve...;)

Hi Peter, unfortunately a system like that which only takes parallel slices will only really work well with certain kinds of shapes.

Anything that has protrusions in it can lead to this kind of a problem:





Notice there how the protrusions caused a kind of bifurcation or branching in the profiles, at one point there is one large profile and then the next one up suddenly has 2 small islands. That kind of thing is not good to try and do with Loft.


If you want to convert Sub-d to NURBS, probably the best way to do it currently is to use T-Splines, which can convert the Sub-d control cage into a set of NURBS surfaces.

Otherwise, if you have frozen smoothed polygons or something like the result of a digitizer or scanner, there are various specialized programs that are focused on this kind of reverse engineering task of fitting surfaces to polygon data. Often times they work by doing some steps to trace out different curves on the mesh data, then those curves form a kind of network topology and boundary lines for fitted NURBS surfaces. That kind of method allows for more general purpose shapes to be handled than planar slicing.


Some of the programs you could check out for this:

Raindrop Geomagic: http://www.geomagic.com/en/
Rapidform: http://www.rapidform.com/
headus CySlice: http://www.headus.com.au/

A few different ones for Rhino are listed here: http://www.rhino3d.com/resources/?category=13


It can be a kind of specialized and finicky area of stuff though... it tends to involve quite a lot of work to make software that does a good job at this, so it is probably not going to be very feasible for me to do it built into MoI directly anytime soon.

- Michael

Thank you Michael !
I will look into it !
Peter

There's a some stored wisdom about doing reverse-engineering with Rhino on their wiki:
http://en.wiki.mcneel.com/default.aspx/McNeel/ReverseEngineering.html

Specifically the Scan, Cleanup, Remodel whitepaper linked at the bottom is pretty good.

However, if you're coming from SubD, it's probably easier to convert the control cage directly to Nurbs rather than getting a subdivided model and doing reverse engineering. It'll be a lot faster, generally better looking, and your Nurbs will have the same isocurve flow as the original SubD.

There are a few programs that will let you do the conversion from the control cage directly to Nurbs. There's Maya, Catia's imagine and shape, and T-Splines for Rhino, that I'm aware of.

I'm a developer for T-Splines, so I'm a bit biased, but I think you'll get better output from any of those than from manually reverse engineering.

Hi Tom. Amapi can do it too, but it always died on me at around 100 polygons. On the contrary, I once converted a mesh in early TSplines beta with more than 80,000 faces. That was before you added the fancy smoothing code... :)

jonah

Thank you for the links and the answers to my questions !
I will look into it ...
Peter