Flippin' normals when splitting a curved surface?! (Sorry, can't think of a better title)

Hi fellow Grasshopperers (although this question is more in the domain of Rhino),

So I've got an interesting question, in the sense that even my teachers did not know what to do about this.

I have this curved surface, and a circle packing (consisting of 40.000 circles...) that was made in Kangaroo is projected onto the surface.

Now what I wanted to do is split the surface with the circles, so that a in my opinion very interesting 'net' structure emerges. And it worked, taking Rhino about 18 hours to do it... (　ﾟдﾟ)

The result of the split, however, has a peculiar thing to it: there seems to be an extra gray plane running through the object. At first I thought it was just a graphical glitch, but it does show and render in every way and it has the interesting property that it seems to go from one side of the net to the other at the "inflection lines" (i.e. places where the surface of the net changes between "concave" and "convex", if you understand what I mean). Oh, and when I select the object, only the 'net' portion turns yellow and there are no isocurves running along this extra face.

Here is a picture of the situation:

Does anyone have an idea as to what might cause this (and of course: how to fix it :D)?

My professor says it probably has something to do with the normals of the surface flipping at these 'inflection lines', which I guess would make sense, but that still does not give me the root of the problem or how it is solved... So I hoped someone in this very advanced community may have something to say about it?

In case anyone wants to dive into this, I have the file uploaded on my OneDrive, because it's quite big (64 MB zipped). It can be downloaded through this link:

https://1drv.ms/u/s!Aid04JjWDIAtvVFQau98yLGMx8qu

Anyways, thanks a lot in advance! I sincerely hope that this will further improve my understanding of NURBS or whatever is the topic here ヽ(*≧ω≦)ﾉ

P.S.

It also seems as if the gray thing disappears when I trim the surface, but then the whole thing shows up as just a 'wireframe' of the object (but it is still treated as a surface):

Replies to This Discussion

Permalink Reply by Arwed Gollner on October 6, 2016 at 6:56am

Hi there.

It sounds like a tesselation problem of the resulting surface. I haven't looked into your file but I have a few questions. Maybe that could lead to a different perspective.

Have you tried to split the surface in GH or in Rhino with baked geometry?

Have you tried to do the splitting only with a portion of the surface? (=dividing the base surface)

Have your tried to rebuild the big surface and to use that as base?

Have you tried to do the splitting in a different software? (for example Alias does the trimming of this kind of thing a bit faster)

I have had a similar problem with a splitting operation in Alias. I never learned the reason but I solved it by dividing the surface and splitting the projecting geometry. It appears some software tools have troubles to do trimming operations on the base of periodic curves, so why not split stuff into two halfs first.

Maybe that helps. Good luck

ps: your could also try to trim the surface not with projected curves but with cylinders on base of your circles. Do a boolean operation with that instead. I think thats faster than the super slow "split surface" component in GH

Permalink Reply by Jiri Swen on October 7, 2016 at 1:38am

Hey Arwed,

Thanks for all the ideas, I'll try them (although I doubt I will be able to test them all before having to hand in the assignment...) and let you know if something worked!

Permalink Reply by Jiri Swen on October 7, 2016 at 1:38am

Hi Tom,

Thank you for all the tips! I already did use a couple of these (i.e. doing it in rhino, using split), but I'll definitely have to try out a few if not all of these. Hopefully I'm even able to fix the problem (＾ν＾)

Permalink Reply by Jiri Swen on October 7, 2016 at 5:56am

Okay, so remember that postscriptum saying that when I trimmed it the issue disappeared but didn't leave any mesh at all?

Well, so I trimmed it a second time (trying to cut out larger holes) using an extruded curve and after that it did output a proper 'surface with holes in it'...

Anyways, your help is still greatly appreciated and I've learned some new things about Rhino again (δ´ω｀)

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