May I ask why sweep tool worked differently to what I programmed? Thank you

Hi all,

I’m currently learning grasshopper, and I’m trying to make a line sweep along two rails. However it seems the geometry sweep towards different direction. Could you take a look and let me know if you happen to know the reason please? Thank you very much!

Henry (19.6 KB)


Is Sweep really the best tool for this? It looks like Boundary Surface might work?

It’s the “usual” problem with sweep.
It doesn’t work like in rhino, you need to check the position of your section in relation with the start of your rails.

In your case, just flip the rails:
2020-05-25 16_22_35-Window

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Check this as well. (20.2 KB)


Ah, I see now what is intended. If you plan to do both sides, it looks doomed to fail with a gap at both ends. The solution would be to extend the curves, mirror the surface and then split both surfaces where they intersect at the ends. (a boat hull)


Thank you very much! May I ask if there is any toll I could use to check the direction of curve in this case?

Thank you and thats exactly what i’m trying to do! It’s a much better solution than mine :slight_smile:

You can always attach an End Points component to any curve (or list of curves), then attach a Point param to its ‘Start’ output to see only the start points of each/any curve.

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Thank you very much!

Hi Joseph, may I ask what is the easiest way to split a surface with another surface please?

Well, there are several ways. You can get their intersection curve(s) and use that to split a surface. You can intersect a surface with a plane, use the curve(s) to split the surface and then mirror the surface to create the other half of a “boat hull”. You can split one brep with another. It all depends.

This is a very ugly boat but illustrates the idea of using two curves to create a hull shape: (14.7 KB)

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Or with each other: (13.4 KB)

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