Hello friends, I have a 3d slab brep, and the final goal is to extract the outer boundary of the top of the slab. I’ve managed to get the top surface as a single polysurface but I am not sure how to proceed. Files are attached. Any help is appreciated! [image] [image] BREP OUTER EDGE QUESTION2.…

[image] Yiming C: …get all surfaces…get all the top-facing ones… Understanding this isn’t about the outer edge/boundary anymore. See if this helps: special-uncap.gh (500.7 KB) [image] [image] *From here if brep outer edges are needed simply ‘Brep Join’ both sets (top/bottom) and use ‘D…

This has worked for me. [image] Outer boundary curve from Breps Grasshopper You could also use Mesh Shadow. [Canvas at 22;15;34] makeBuildingCut_re.gh (49.7 KB) Closest functionality I’ve found to the Rhino curve Boolean method

Thanks! I’m looking to extract the 3d boundary, is there a way to do it?

maybe ‘dupborder’ :slightly_smiling_face:, that looks like a mesh though and you want .gh :sweat_smile: I’m sure there’s a way though, some our gh buddies will show up soon :smiley: me jus doin’ what I can to keep this at top of list :innocent:

Welp, would like to edit the main post but it won’t let me… Correction: I thought I got it, but I didn’t, so I failed to get the top surface geometry. I planned to get all surfaces’ normal and only keep those with a positive z value to get all the top-facing ones, but the result is not consistent; m…

i don t see myself as a gh buddy nor a spaghetti-master … but lets try to help: OrientationIsReversed Please note, that BrepFaces might have a reversed orientation compared to the underlaying surface. This property is as far as I see not exposed to vanilla Gh. This is why my definition is lazy: i …

Absolutely wonderful! I guess using the 0.5,0.5 uv point for surface normal is more efficient than ‘surface centroid - closet point to brep - surface normal‘?

Hi Tom, this is another great solution, thank you so much! I figured out my previous script was using the ‘closest point to surface’ instead of ‘closest point to brep’, which is causing some normals to be misinterpreted as the opposite direction.

[image] Yiming C: using the 0.5,0.5 uv is not correct - as the domain of the surface is not addressed “normalized”. this is why i use the grouped/named “domain center” section. but yes addressing a point on a surface by u,v is much faster then calculating area centroid and closest point. bu…

[image] Tom_P: i don t see myself as a gh buddy nor a spaghetti-master Can’t tell if derisive or not. [Screenshot 2025-11-25 184657]

i am refering to [image] CAD CAM CNC: our gh buddies and [image] Tom_P: spaghetti-master ( i used some c# script to solve the question) [image] What's your largest Grasshopper script? [The Hall Of Shame] Grasshopper I’ve got one script sitting …

How to get a polysurface's outer boundary?

Grasshopper

Tom_P November 25, 2025, 5:11pm 12

i am refering to

and

( i used some c# script to solve the question)

and i love this forum and all members.

have a nice evening - cheers - tom

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How to get a polysurface's outer boundary?

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