Hi Lars, it's quite difficult to do a good quality conversion from polygons to NURBS objects.
Polygon data has everything all smashed up into little planar faceted bits.
NURBS objects has things structured as large smooth spline surfaces.
It's possible to turn a NURBS object into polygon mesh data by taking the big smooth NURBS surfaces and dicing them up into little pieces. Think of this as similar to making applesauce by taking apples and smashing them up.
But it's quite difficult to go the reverse way and try to reform large surfaces from already diced up data - that's sort of like trying to start with applesauce and then trying to reform whole apples again from it.
There are some special types of reverse engineering tools that do that kind of stuff but they're complex and expensive. See here for some links to some of those:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4829.3
http://moi3d.com/forum/index.php?webtag=MOI&msg=1901.9
If your object has a relatively low polygon count, it can be possible to convert it by making one trimmed planar NURBS surface for every polygon face - to do that use the Obj to 3DM wireframe converter as described here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5135.5
However, that kind of "one individual little trimmed NURBS surface for every polygon face" type conversion won't work very well with heavier polygon counts because NURBS objects are not intended to be made up of so many separate little fragmented pieces, they're intended to have bigger pieces of the model made up of large smooth surface patches. There is some more overhead to the way NURBS represent surfaces and trims so having a zillion little surfaces creates a very heavyweight object in that pseudo-NURBS form ("pseudo" because it's all faceted and not actually smooth). See here for more description on that:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5135.13
So basically it is technically possible to do that conversion but it's often not a good idea. Usually the conversion only goes the other direction, from NURBS to polygons.
- Michael |
