Lofting periodic and closed curves

Newbie to GH hoping someone has a simple answer to this problem.

I have a definition where two 3D curves are generated from a set of points (as control points and as an interpolated curve). Then those curves are projected onto a flat plane below. I want to loft the flat curves to the 3d curves, but GH won't do it. It says it has NULL values. Is it because the original curve is periodic and the flattened curve is not?

If I bake the curves, I can loft them in Rhino. Rhino also has a "MakePeriodic" command, but I can find no corresponding module in GH. Or maybe the problem is something else?

Tags: loft

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Permalink Reply by Luis Fraguada on June 23, 2009 at 12:29am

If you can post a simple version of the definition, we may be able to help. Could be an issue of how the curves are organized.

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Permalink Reply by Dirk Anderson on June 23, 2009 at 1:01am

If you flat plane is a standard XY plane (no rotation), one way around this is to take the original curve vertices (the point lists), pull those to the plane and interpolate though points.

Alternatively you can use the original point lists, decompose and re-create a point list using the XY (from point) and Z from plane height.

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Permalink Reply by Damon Caldwell on June 23, 2009 at 2:52pm

OK. Thanks! Your first suggestion worked. Instead of projecting the curve, I projected the generative points and then made the new curve from the new points.

So this problem is solved, but I still wonder about the larger issue. What exactly is the difference between a closed curve and a periodic curve, and why don't they loft in GH, given that they do in Rhino directly? And is there a way to transform one into the other, like in Rhino? Just in case the issue comes up again.

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Permalink Reply by Damien Alomar on June 23, 2009 at 10:28pm

First of all, periodic curves are all one curve span, were as non periodic curves may be made of pieces of other curves. The places where those curve pieces are joined result in kinks at those points. After that, the main feature of periodic curves is that they are curvature continous (C2) at the seam of the curve (the start and end points). This is achieved by changing how the control points actually influence the curve. In a normal curve, the position of the start and end points are influenced only by that point and not any other point in the curve. With a periodic curve, the influence of each of the points "wraps around" in regards to the start and end of the curve. Therefore, every portion of the curve is influenced by multiple control points.

As to why these don't loft directly in GH I don't really have much of a clue. My very basic test here works, but I'm not sure exactly what you're dealing with.

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Permalink Reply by Damon Caldwell on June 23, 2009 at 11:40pm

Thanks for the detailed explanation. That makes it clearer.

I have attached the definition that is not lofting. Maybe the problem lies elsewhere than the curve difference. I did rewrite it based on Dirk's advice, and that version works, but all curves in that scenario are periodic.

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