A PolySurface in Rhino is considered as single surface in GH


I have a very werid problem today. I extruded a surface in grasshopper, and then deconstructed it. But the faces I got are only 3 surfaces instead of 6. The “untrimmed surface” is considered as a single surface in GH. However, when I baked into Rhino, it showed as a polysurface, and can be exploded into 4 surfaces.

I also offsetsurface by using pythonGH, but it still happened.

question.3dm (70.4 KB)
question.gh (10.5 KB)


I see it too. Looks like a bug in the Grasshopper DeconstructBrep component, or am I missing something here @DavidRutten ?

hi all,
I don’t know if it’s a feature or a bug, but it seems to be because the surface fed to offset is a trimmed surface.
Once you untrim it, the rest behaves as expected.
(for some reason, in this case the trimmed and untrimmed surfaces are the same size)

TRIMMED.gh (10.8 KB)

Hi, Thank you for your reply.
I try to draw a new surface in rhino and trim it. It doesn’t show this feature.
But thank you anyway.

yes, I tried it myself too.
what did you trim your original surface with?
Most probably it was something that looked like a rectangle but wasn’t.
can’t you post the definition that produced that surface in the first place?

In Rhino core the Extrusion object type is implemented as a type of Surface. I never fully understood why this was done, as extrusions are clearly polysurfaces, especially when they have caps, but it was probably due to non-breakage-constraints.

I think the explode component may indeed have to special case extrusions and convert them to proper breps first. Logged under RH-47848.


I reported this before I bought the upgrade to v6 after testing the WIP. I can’t recall currently was it an email conversation or just mentioned it in one of the topics on the forums.

I could not understand the change of the naming. Back then I think it was necessary to explode twice to get separate surfaces. First explode was giving you polysurface.

If this is a step-by-step transition to feature-based modeling, (like inside catia). First you should think of creating container objects that contains the separate surfaces of the polysurface, then transition this into a feature.

The problem seems to be that the trimming curve, despite having four kinks, is a single edge. This should not have happened using regular Rhino commands/api functions, but of course there may be a bug. @chi how was the original trimmed surface created?

Hi all,

Sorry for the late reply. I try to simplify my script, and I find out this may be either caused by “offset on surface” or “surface split”. It seems only happened to plane surface.

question_2.3dm (78.3 KB)
question_2.gh (12.9 KB)

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