"Isotrim" untrimmed my trimmed surface

Hi guys,

I want to divide a curved hexagon surface into 5 by 5 pieces. But instead of the trimmed hexagon surface (green in screen shot), “isotrim” applies to a 4-egde surface (red in screen shot) which is bigger. Can someone please tell me how to trim the surface again (or maybe in another way to divide the hexagon surface)?



M_Huang_project 2.gh (63.4 KB)

The reason this is happening is that when you “trim” a surface, the rest of it isn’t really gone exactly, it’s just disappeared. Trimmed edges are different from untrimmed edges in Rhino in this way - you could hypothetically run the “untrim” command on any one of them to get it back. If you bake something like this and then turn on the points, you’ll see all of the control points even for the areas of the surface that aren’t visible.

In your particular case, you should bake out the hexagon, use the ShrinkTrimmedSrf command on it (which, to the best of my knowledge, will just attempt to refit the parameter space. See Rhino Level 2 Training manual for more details). Then reimport the surface. Since your surface has more than 4 edges this might still result Isotrims off of the edge of the hexagon. In that case you will have to manually create a single (untrimmed) surface in Rhino that approximates your shape and desired UV arrangement (since Isotrim is following the UVs when it cuts. You can check what the general UV space looks like by turning on control points). I’d recommend Loft, but pick your favorite surfacing command.

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The Pufferfish plugin contains a Shrink Trimmed Surface component.


its actually quite easy,use Isotrim and copy trim

isoTrimTrim.gh (18.9 KB)


Hi Dan,

Thank you so much for the instructions! Yes you’re right, the 4-edged surface is smaller but still there because of the UV arrangement.

Actually I created the hexagon directly from 6 control-points in the space (not a 4-edged surface trimmed with 6 edges). Do you know if there’s a plug-in to get the triangular construction lines? I think the UV arrangement is the problem.


Thanks Ethan! it shrinks the 4-edged surface but it’s still there. I’m still looking for the exact tool to find the triangulate construction lines.

Thanks Tom! But actually I’m working with the big hexagon surface, not the panelization of the 4-edged surface.

It’s not possible. Nurbs exist always as untrimmed 4 point surfaces. You can trim them but any evaluation (like an iso) will always be of the untrimmed underlying surface. Best you can do is use the iso’s, then trim the entire result with the trimmed surfaces boundary.


Thanks, that sounds reasonable!

How to do it? can you add example?