Sweep 2 Grasshopper simple problem

I know I could use loft to do this but I need to incorporate sweep 2 to add some functionalities later. However this simple definition doesn’t work. The same curves make a perfect surface in regular sweep2 in Rhino. What do you think is wrong?

Sweep2 Problem.gh (5.0 KB)

Left: GH Sweep2 Right: Rhino Sweep2

Works fine in Rhino 7 WIP, broken (per your image) in Rhino 6. Very weird.

don’t know if it helps, but in Rhino_6 the sweep2 looks like working if the original curves are exploded

Sweep2 Problem_re.gh (9.7 KB)

@Joseph_Oster I checked and yes it works in 7. Probably it was a known issue that’s been fixed. I guess I’ll stick with the 7. Thanks.

@inno That’s a good idea, though it may mess up when we want to send multiple rail1 and maultiple rail2 curves, right?

it depends on what you are trying to do next

I agree… Still I don’t like to get around of a very basic feature, that’s why…

Is Sweep2 with straight line sections really any different than just lofting the two rails?

P.S. Here’s the difference in R7 WIP. One face instead of four? Surface area is the same.to 5 decimal places.

No, it’s the same thing… I just want to understand why the problem occurs.

Don’t you think it’s unusual to get one face?

Yes I do, never seen that before but I generally don’t try to use “kinked” rails with Sweep.

Why not? It works great on corners imo.

Not always. Recent threads on segmented rail Sweep failures:

Oh wow. It works in R6 if you reverse the list of sections.

Sweep2_2020Dec6a.gh (7.2 KB)


Wow! But why?? :smiley:

Direction of the rail curves must match the sequence of the sections (or vice versa). Apparently R7 doesn’t care about that.

So, does the first cross section curve should be the one that coincides with the seam of the rail curves?

By the way, reversing the list only work with 2 cross sections. More than 2 don’t work.

In R6 (and R5) yes, the first section would be at the start of the curve, last at the end.

You can waste a lot of time trying to understand and cope with GH quirks like this. Or just use Loft and move along to the next phase of the project. I recommend the latter.

Maybe you have missed but my first sentence was “I know I could use loft to do this”. So I know that I can use loft but thanks for reminding anyway and for the brainstorm. Apparently you don’t know either why this happens. That’s what I was wondering and not how to get around it.

Fine. Grasshopper is chock full of weird quirks and bugs that collectively must consume thousands of man-hours in wasted effort trying to understand “why this happens”. Even when bugs are documented they may not get fixed for many years. You’ll get academic explanations and excuses but if you want to get real work done, knowing how to get around them is key.

That’s what I already did while waiting for an answer. It’s valuable to understand these things because it gives me a deeper insight. Thank you for your help!