How to do this simple Nurbs shape in SubD precisely

Rhino SubD
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1

Hello!

I am new to this forum. Recently have been practicing SubD modeling. I found SubD is very powerful for organic shape. But I couldn’t find an effective way to model something like making a clean concave cut in a cylinder shape like the image below. The left model is a very simple Nurbs model that I want to create in SubD approach. In my SubD model, like the right one, the surface around the edge of concave shape is slightly distorted compared with the model generated with normal nurb approach.

Is there any advice for me to achieve the shape on the left in SubD ? Thank you!

image subD
image subD1611×1425 344 KB

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Sub-D modeling has some inherent limitations including:

  • Circles, cylinders, spheres and similar shapes cannot be modeled exactly.

  • At edges and creases the adjacent surfaces have zero curvature in the direction normal to the edge.

  • Trimmed surface are not possible. (I don’t know if this will change.)

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Thanks for your reply David. In the case I posted above, do you know how I can work around these limiations and get a better result?

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To what end? The sort of thing you’re trying to do here is precisely why NURBS still rule the CAD world. You are absolutely not going to discover some simple workflow tip that makes Subd as good for that stuff, it doesn’t exist, people have been trying to do it for 20 years.

5

model it in nurbs and then quadremesh it into a subD, with a lot of faces. make sure to take advantage of the symmetry functions in quadremesh.

That’ll give you a subD that gets closer to your nurbs shape. The fewer faces it has, the less accurately it will resemble the original. The more faces it has, the more of a pain it will be to integrate it into a larger subD object that has less precise parts.

For some perspective on the limits of SubD: A symmetrical SubD tube with 8 horizontal faces is under 1% out of circular. Anything with fewer horizontal faces will be visibly non-circular to the trained eye. This kind of inability to get as precise as Nurbs in some cases will amplify the more complicated the object is, with creases often being a major factor.

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Thank you Max for your suggestion. I will definitely give quadremesh a try!

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i have been using CAD tools in my ID design job for 10+ yrs and it has been great. Recently i found some of my colleagues who has been using Rhino integrated the SubD function into their design process and they are a lot faster than me especially in the early concepting phase. With what you said, it seems like it probably cannot replace my CAD tool in my job. But it probably can be a quick way to brainstorm and try new ideas (like sketching) in early design phase?

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