Edit surface control points and maintain surface edges

Hi guys,

Is there a way to edit a surface’s control points and maintain its boundary edges fixed in place?

I have a polysurface and I want to edit some of the surfaces. If I edit some of the control points of some of the surfaces then the edges between the edited surfaces do not match with each other and I am left with naked edges that were originally not naked.

Thanks for the help.

Best regards.


Hi @Leonardo_Aguiar
In most surfaces, if you do not change the control points of the surface that are “on the edge”, then edges of the polysurface should continue to match.

Hi Rajaa,

Thanks for the answer.

I think I have surfaces that are different from most surfaces.

I am not selecting the edge control points and the edges are moving. I have tried to rebuild the surface and took the degree magnitude down to 2. The problem still persists.

Can you please share the surface?

Are the surfaces trimmed or untrimmed?

The surfaces are likely trimmed, and this is one common case when edges become unaligned when editing control points.

Hi Guys,

The surfaces have been trimmed and the control points have been shrunken.

Attached is the file. I have to edit all of the red surfaces to be inside the gold surfaces. I was planning on doing this by editing control points but I need the edges of the red surfaces to maintain their positions.


Best Regards,

Leocobra final for rhino forum.3dm (15.2 MB)

Yes, with trimmed surfaces, it is challenging to control where the edges will be located, when the surface is modified with control points.

It is best to create your surfaces so that they are within your boundary to start with and are untrimmed at least at the bottom edge. That is said, using the surfaces you have, here are a couple general suggestions of how you can modify to align:

  • Re-create the surface so that the bottom edge is NOT trimmed. This will help keep it in place when you edit the surface later. Attached an example. cobra final for rhino forum_RI.zip (6.9 MB)

  • Modify the surface, even if the bottom edge does not align. Now recreate the surface using Patch where you select the border and the “original bottom edge curve”. You might need to do a follow up editing, or adjust the patch parameters to get what you need.

I hope this helps.

Hi Rajaa,

Thanks for the help.

The surface you numbered as number 1 is behaving how I would like. The surface you numbered as number 2 isn’t.

What did you do exactly?

Thank you so much.

  1. surface simply untrimmed your surface (use command UntrimAll).
  2. Used the extracted curves to generate a new surface from loft that is then trimmed back to your original boundary. The idea was to align the control points along the bottom edge.