New Problem using Sweep 1 rail with 2 different sections curves


Yesterday i made a post about the function "Sweep 1 rail " in Grasshopper, using only one curve for the section.
Now, i have a problem, as i am trying to sweep along one rail, but, with 2 different curves for the section.
At the end of the definition, the function is in red. Even if it makes the sweep, this is not the sweep i have making this in Rhino, and i don’t know why. I think this is something about the direction of the 2 curves, but, i don’t know what to do.
First, i join the definition in screenshot. I tried too, in Grasshopper, to make a second defintion, changing the “End Points” function, putting in output “End”, insted of “Start”. In this case, the definition is in grey, but, i have the same shape that in the first case: it seems there is a rotation inside the shape, due to the bad direction of one of the 2 curves.

Here i join the screenshot of the shape made with the red definition in Grasshoipper, and baked in Rhino.
IT seems there is a rotation of the section.

And here, i join the screesnhot, showing the Sweep along 1 rail, but, in Rhino.
As you see, this is ok, there is no toration of the sections.
But, i don’t know how to do it in Grasshopper, can you please help me?
Thanks so much :grinning: :grinning:

You almost replied yourself.
Look at your first pic:
the start point of the sections are on the same side, but the 2 sections are on the opposite sides of the rail!
Your left section should have a start/seam point “mirrored” to the one of the right section!
Rotate 180 around Z one of the 2 sections.

Also, you are using 2 rails at once, that doesn’t have sense.

As usual, attach your file (a minimal version of the problem, with internalized data) to get help faster.

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I can’t tell from this small image of your code (a GH file is WAY BETTER!) but this looks very similar to yesterday’s thread:

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Thanks so much for asking me!
I thing i tried to mirror, but it was the same, but maybe i did not make this well.
I don’t understand when you write that i am using two rails at once, it seems i have two sections, and onlyu one rail, but maybe i made a mistake :grinning: :grinning:
I put now the attached grasshopper file with internalised data
Thanks so much :grinning: :grinning:

This is the grasshopper file with internalised data.
As you can see, the sweep function is red, and the sweep is bad, because it seems there is a rotation of the sections and i don’t know why :grinning: :grinning:
Thanks so much :grinning: (7.8 KB)

Thanks so much but now, i have two different sections, and i don’t know why i have this problem.
I just come to share the grasshjopper file, with internalised data.
Thanks so much :grinning: :grinning: (10.5 KB)


Hi Joseph
Thanks so much for the quick answer, but, in your definition, you have the ame shape that i have using grasshopper, that has a rotation around the sections that i don’t want to have. :grinning: :grinning:
Here, i join you the Rhino File and the shape i have using the same curves and sections, but using Sweep 1 rail in Rhino, as you can see, there is no rotation around the profils of section, and i don’t know how can i have this, using Grasshopper?
Thanks so much :grinning: :grinning:Rhino26fevForum.3dm (710.0 KB)

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Gh is not as kind as you might think like you did in rhino, but you can get a hint from this… (10.1 KB)

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I had no idea what you are talking about… But looking closely at the geometry, I can see the brep in the .3dm file has substantially more volume than the Sweep 1 result: 2388.73 vs. 1795.84, as you can see in this top view. The white curves are the cross-section of the .3dm brep:

The station curves appear to be identical, though for some reason the smaller cross-section of the .3dm brep fails to return a valid Area:


I’ve looked at this many different ways but can’t explain what’s going wrong, sorry. I’m guessing there is something about the way the section curves are created?
P.S. I see that @HS_Kim recreated the smaller section curve by mirroring and scaling the larger one.

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Hi Kim!
Thanks so much for the answer!
I will try this definition in few minutes! :grinning: :grinning:
The thing is i don’t understand exactly the way the function “sweep along 1 rail” works in Grasshopper, and why do we have to put the function “Seam”, and I don’t understand very much in Grasshopper the function “Seam”, as it is so eazsy to sweep along 1 rail into Rhino, but not in Grasshopper, as i don’t know too, why do we have to create a list of item in Grasshopper? :grinning: :grinning:
But if i am doing this, it is because, after, i want to be able to parametrize different curves directly inside Grasshopper! :grinning:
Thanks so much! :grin:

Seam moves the start/end point of the rail curve so that the first section curve is at the start of the rail.

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Hi Joseph,
Thanks so much for your answer!
Yes, the brep in rhino file has more volume.
I don’t know what happened with the small curve.
I hope that the function “sweep along 1 rail” in Grasshopper will permit me to sweep others curves, because, at another step, the aim is to sweep a lot of curves… :grinning:
Thanks so much :grinning:

Thanks so much!
In this way, if we have, for example, 4 sections, then, we only have to put the seam function for the only section curve we want to be the beginning of the rail? (and the end too, if we have a closed sweep)? :grinning: :grinning:

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Hi again!
Thanks so much! :grinning:
As i don’t understand very much the function of “list item”, i tried to do your defintion, without the function “list item”, that i put here.
But, could you please explain me, how this function is working, because, what i don’t understand, is that in input, we have two closed planar curves, and, at the input, it is written that we have only one closed curve. But, It seems that, in the output, from one side, we have the end point of the bigger section curve, and, from other side, we have the bigger section curve that enters in the input of the function “mirror”. So, i don’t understand, how, from the function “list item”, we can choose only the bigger section curve, and not choose, the smaller? :grinning:
Thanks so much (9.7 KB)