After searching for a suitable replacement for curved/membrane-like structures with NURBS, I looked into subD and watched a few tutorials in the hope of learning how to use them for my use case.
So, below, on the left, is my basic NURBS surfaces as a starting point. I would like to model a subD box to exactly follow the a x b (in red line) and all other lines to be “soft”.
To achieve the red edge, have made the “diagonal” creases, but it is then similar to my NURBS model and not soft. Think of this form as half of a pillow or a pressurised membrane.
Since I am a beginner, can you please give me some pointers on how to achieve this?
Thank you!
This kind of vertex, where the limit surface vertex position is exactly the same as the control net vertex position, is called a Corner vertex. There needs to be at least 2 Crease (or boundary) edges going to a Corner vertex, and any vertex where 3 or more Crease edges meet is a Corner.
You can learn more about the rules of SubD component tags in Rhino in this document. Note that this is advanced technical documentation that I’m just offering as a reference in the hope you find it interesting, but reading and understanding it is not required to work with SubDs in Rhino and there are much better training materials in our resources links if you are just getting started.
Geometry, independent of any math used to model a surface, has three alternatives for a surface which has a sharp corner in the edges:
A) The surface is smooth and the edge curves define the surface normal direction at the corner. If the edge curves are planar then the smooth surface will be flat at the corner.
B) A crease in the surface which ends at the sharp corner.
C) The corner is the vertex of a “cone” and the curvature is infinite at the corner.
These alternatives are because of how the geometry of surfaces work, not limitations of NURBS, SubD or similar. They apply equally to physical surfaces.
InflateCrv in V9 WIP produces a type A surface, flat in the corner.
Read my post above. You have encountered a limitation of geometry. The “workaround” is to change your design intent to something that is feasible in geometry. Which of these do you want:
Since my biggest issue is with the corner, I tried to tackle only the corner.
I tried NURBS first. So if I run a curve network command on the corner curves and manipulate the geometry vertices a little bit, I get pretty close to the 2 trimed surfaces (see example 3 from left to right).
Is this surface geometry something that could be translated into subD, or again, the edges meet at 90, therefore, no, it is absolutely not possible? Is there a transformation command I could try?
The last example on the right is a lofted subD surface that I tried to somehow manipulate to make a sharp edge.
Do I understand subD correctly? If a surface is possible in NURBS, does that mean it is also possible in subD?
