# Converting Sub-D < > Nurbs : points/vertices/edges/surfaces selection?

Hi,

I’m beginner with Rhino 7 and I have few questions about the behaviors of Sub-D and Nurbs.

I followed this tutorial wich shows that the wright way to use Rhino is as followed : MESH > SUB-D > NURBS, that’s ok for me.

But I show here Screenshots of few situation I don’t understand.

Image 1 : Why the last Nurbs are more subdivised that the first one, and why for this one the control point can’t be selected ?

Image 2 : same questions, why second sub-d is more subdivised ?

Image 3 : why after boolean union between two sub-d spheres, I can’t select the control points

Hi,
I would like to highlight my first message.
I wonder why subdivision changed by converting nurbs to sub-d and conversly, or by applying boolean union (see screenshot below) ?

I am not sure what that means - there are many ways to use Rhino. Converting back and forth between Nurbs and SubD is really not a good one though. SubD and surfaces/polysurfcaces and meshes for that matter are all different and using one or the other really depends on the goal you have.

Booleans of SubD objects converts them to polysurfaces. These do not show control points.

-Pascal

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Hi @pascal

Thanks for return.

I also don’t know what that means, but I found this information in few tutorials (?). I tried this way with the tutorial mentionned above and it works well.

Ok, I understand why I obtained subdivision (in other words, is thus that correct to say that booleans of Subd objects converts them to Nurbs ?)

When you make 3D objects, your first step is generally making a few curves which define these 3D objects. Your second step is extruding or rotating or sweeping (along rails) these curves to make surfaces. If the 3D objects have organic shape (e.g., dinosaurs), the best surfaces are SubDs. If the 3D objects are machine parts, the best surfaces are polysurfaces. Sometimes you make SubDs and you convert them to polysurfaces before making holes. Your third step is replacing sharp 3D edges with fillets. Your fourth step is either applying materials or converting the 3D objects to meshes. If you want to make pretty picture, you apply materials (grass, wood, gravel, displacement, etc.). If you want to fabricate the object using 3D printer, you make sure that there are no holes in the 3D object before you convert it to a mesh.

Watching video tutorials is the best way to learn basics. If I were in your shoes, I would download tutorials from these websites:
Vimeo (Robert McNeel & Associates): http://vimeo.com/rhino/videos
When you have the videos in your computer, watch them slowly and pay attention to command names.
Rhino 7 commands are listed here: Command Quick Reference | Rhino 3-D modeling
Rhino 7 user interface is explained here: The Rhino window | Rhino 3-D modeling
If you use Rhino, you will also use its plugins and other graphical programs. I recommend Affinity Photo (Photoshop clone, price = \$40).

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Many thanks for this answer, this is very clear for me and very interesting. I should apply these advices soon

Please, could you send me few complementary informations on this step :

And if possible, I would like to obtain your confirmation on this sentence “is that correct to say that booleans of Subd objects converts them to Nurbs ?

Yes. If you try to do a solid boolean operation on two SubDs, or one SubD and one NURBS polysurface, Rhino will first convert the SubDs to NURBS polysurfaces then run the boolean operation.

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Most machine parts are made by cutting steel. The cutting tools make sharp edges. Sharp outer edges are dangerous because they may cut human skin. Sharp inner edges are dangerous because they concentrate tensile stress. When the part fails, the failure usually begins at sharp inner edge. The sharp outer edges are removed by filleting (rounding) or chamfering. (Electrochemical machining is perfect method of filleting the sharp outer edges). Rhino has several commands for filleting and chamfering NURBS surfaces and polysurfaces: FilletSrf, FilletEdge, VariableFilletSrf, BlendEdge, BlendSrf, VariableBlendSrf, ChamferSrf, ChamferEdge, and VariableChamferSrf. Sometimes these commands fail. Loose tolerance, misplaced surface seam, very short edges, and tangent surfaces are the main causes of failing polysurface fillets.

SubDs generally do not have sharp edges. Crease command can make sharp SubD edges. Bevel command can fillet and chamfer sharp SubD edges.

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Hi @pierrec Hi @andrew.nowicki
Thanks a lot for these complementary informations.
This post help me well to go on learning Rhino.