Outer boundary curve from Breps

I have a bunch of buildings, and want to get the outer silhouette to perform a set of split operations on the terrain surface.

I haven’t seen any good solutions to this problem, am I missing something?

I’m looking for closed curves like the green line (without the offset).

I was not able to upload the Grasshopper file, it’s only 67KB, here is a link to it.

Thank you!

Hi @julian.riise,

If you bake your geometry, back into Rhino, and then run the Silhouette command, do you get what you are looking for?

– Dale

If you can get curves rather than closed Breps, there’s a tool in Grasshopper called Region Union. I’ve uploaded an example with some internalised Breps.

The script takes your Breps, finds the face with the lowest z-coordinate center (so the bottom face), explodes it into its curves, joins the curves, then takes all the curves and uses Region Union on them to produce the effective combined outline of your Breps.

Hope that helps!
RegionUnion for Julian Riise.gh (14.5 KB)

(Edit - note that there may be a more efficient way to always select the bottom face; this is just how I’ve been doing it for a while - main point though, curves → Region Union)

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Elegant solution! I had some issues with the Region Union component, it took forever to process, even on a small sample size of buildings…

I don’t know if you had that issue? I’m ultimately running the definition on ~350 buildings.

Thanks for your help!

You could also use Mesh Shadow.

makeBuildingCut_re.gh (49.7 KB)


makeBuildingCut_2022Feb6a.gh (45.6 KB)


I didn’t have that issue, but I also only tested on the boxes I posted there.

I did a quick test and arrayed out the sample boxes. With 3000 closed polysurfaces (so 600x the 5pc example I posted) the script took 2.8s. With 9000, it took 14.7s.

(500 closed polysurfaces)

(9000 closed polysurfaces)

I then took the Mesh Shadow example posted by @HS_Kim above (only up to the Mesh Shadow component, not the surface projection) - the 9000 closed polysurface example completed in 6.7s, so I’d say that’s an even more elegant solution. In fact, 18000 polysurfaces only took 24.2s on my machine. If that gives you the results you need, I’d say go with that!