Best method to loft this?


I am trying to loft this with all straight sections but I keep getting diagonal seams. Both closed curves have 22 segments (I split one arc on curve 2 to make the number of segments the same). Curve 2 is offset inward so it is smaller than curve 1.

What is the best way to get a nice straight loft on something like this?

Lofting.3dm (322.6 KB)

Looks like an extrusion, isn’t?

The solution could be to loft according to the arcs.

It is not really an extrusion because a lot of the arcs stay the same size while the straight areas taper.

I can get it to work if I explode the curves and pick each one individually, but it won’t work when the curves are joined? I also made sure the crvseam and directions where the same on both profiles. Why doesn’t it work when joined?

Ah, from the image it’s hard to understand.
Would be best if you can upload the file.
Anyway the problem it’s in the node count over the two section.
For example try to imagine a triangle fitting in to a square. How to handle the 4th vertex?
That’s the problem.

The middle way solution it’s to split the sections in two or three sub section and do loft between them. Isn’t a straight solution but works.
Another solution could be to use the Sweep2R that has an option to drive the surface through the sections.

Thanks, I attached the file in the first post its right under the picture. I think I understand what you are saying, but wouldnt the node count be the same if both closed curves have the same number of segments? What am I missing?

Ooops, I didn’t see it.

Yeah, in general term the number of node must be the same but it depends on how you created it.
Generally a circle it’s made by 4 90Degrees arcs. When you trim the circle (arcs) to combine them with other crv some would be made by only 3pt other could be made by 5pt, 2 pieces of 3 pts with 1 shared pt.
Switch on the control point an select the two side to see the pointcount. Probably will be different.

I think I understand what happened. While both profiles have the same number of segments, one profile has more arcs than the other one. An unequal line to arc ratio would cause an unequal node count. Is this correct?

Hmm…They both have the same control point count? 44 each.

Hi hi… ok, I see…
I tried and it works if you change the alignment of the seam. (The two arrows appearing in to the loft)
The problem is the seam alignment at the start of the creation that has to be placed over a node (where two crv join together). Because the different length of the two section the standard alignment fail to ride correctly over the sections.

Always thinking to the most complicated solution :wink:

If you SimplifyCrv and then Explode, one curve has 21 and one has 22 segments- there is one short line segment in the ‘far’ curve that is not on the near one, it looks like, so things get out of sync.


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Sorry for the delayed response I was getting an error when logging into my account.

Skysurfer - From what I can tell, I had both the start points start on a node in the file I uploaded right?

I found that if I just reversed the direction, it lofted properly. Am I missing something?

Pascal - I broke the neighboring arc to give the two profiles the same number of curves and nodes. Is there a better way to do this without altering the geometry or exploding? Should I just try reversing the direction if it doesn’t come out right the first time?

Thanks for the help guys, much appreciated.

Not really - well, you could, in this case, I suppose, Sweep2 a line and AddSlash in a bunch of places - snapping to opposite endpoints on the curves, but I don’t know if that gets you anything really - I guess I’d divide the curves up into a few pieces that match and Join the results - I don’t know that you need to do each curve separately to keep it organized.