Untwisting twisted subD edge loops in a Multipipe with no branching

Dear Rhino family,

How would I untwist the subD edge loops in this image? It is a variable radius multipipe with no branching of an open polyline with 178 points. The radius increases in a cubic fashion with a base constant value, while the polyline kinks a lot.

I wish to crease the side edge loops and delete the top one to create a flat edge as a walking surface for a bridge. However, due to the twisting of the side edge loops to the bottom/top, this is not viable.

untwisting subD edge loops (problem).3dm (548.5 KB)

untwisting subD edge loops (intended outcome).3dm (457.6 KB)

Thank you all so much for the help in advance! Really appreciate it! :slight_smile:

Ariel :slight_smile:

Hi Ariel - you may need to do this in multiple parts, and stitch them together.


To add to what @pascal said, you may also want to simplify your model untill it falls apart, then add just enough detail back to get it to make the forms you want.

remember, you only need 3 edges, or points to make a transition or bendโ€ฆ no more, no less. anything else that should be straight should be one face or edge.

Itโ€™s a bit counterintuitive, but SubD is very much a game of how simple can you make your topology and still get your form, more topology is rarely the correct answer.

see this video-


@theoutside @pascal thank you so much for the answers. I really appreciate them. These are lovely resources. Thanks again! Iโ€™ll let you know how it goes :slight_smile:

Dear @pascal @theoutside,

It worked for me! I realized that I could select and rotate the edgeloops easily, especially by positioning the gumball with reference to the object rather than the Cplane.

Also, I could easily rejoin the subDs back into a closed subD without holes by using the FlatShade mode (โ€œtabโ€) and orienting + scale1D the edges/vertices! :slight_smile:

rotating subD edgeloops! :slightly_smiling_face:

broken subDs that are unjoinable

aligned subDs

well joined subDs :slightly_smiling_face:

fixed subD end outcome

Thank you guys!