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# Strange Behavior From Loft Surface tool

## Question

I use the Loft Surface tool a good bit, but I find it's output to be highly unpredictable.  In the photos below, I'm trying to loft a surface from 3 nurbs curves that have been derived from the same original shape (so all are 3-degree curves, all the same direction).   When I select them, the path indicated before I hit enter seem correct--it's a curve beginning at one curve, passing through the middle curve, and ending at the last curve.  But when I hit enter and go to preview the Loft output, the path changes and now it's as though it's looping around the 3rd curve to connect to the...inside?  I'm not sure.  If I rotate this whole collection of curves into different 3D orientations, sometimes the extent of that weird looping around changes, even though no change has been made to the relationship between the curves.  What the heck is going on?  How do I get it to loft me a surface like the initial preview line suggest it should?

Loft Problem.vwx

## Recommended Posts

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@jbelsky that is an odd one. I haven't been able to get the same thing to happen on my end. Would you mind posting the example file from your screenshots so I can take a close look?

Thanks!

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@jblock, I believe I did in the original post.  It's called Loft Problem.vwx

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If it helps at all, the weirdness shifts depending on which order you select the curves in.  Here below are pics of the same 3 curves selected top to bottom vs. bottom to top.

I

Edited by jbelsky

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Thanks, I missed the file in your original post. How did you create these NURBS? Did you create one by converting a Circle? It appears that the circular NURBS is causing the problem. If I redraw that and make it slightly elongated, it seems to work fine...

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The circle is an oval with equal width and height, so it converts with the same number of vertices as the other ovals.  If you use a regular circle, it doesn't like to align properly.

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I just tried altering the "circular" oval NURB and it doesn't help.  Just makes the distortion more extreme.

Edited by jbelsky

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Is there a way to revert the "Direction" of a NURBS Profile Curves ?

Beside that all start points should align properly to not twist the Loft result,

normally in other Apps there is also a proper direction needed,

similar to drawing Polylines,  CW vs CCW.

That is why, when I do rarely Loft Objects,

I draw the first Profile only and do copies only for the following Profiles

which I will manipulate.

To ensure same direction, same start point location and same number of points.

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17 hours ago, jblock said:

Thanks, I missed the file in your original post. How did you create these NURBS? Did you create one by converting a Circle? It appears that the circular NURBS is causing the problem. If I redraw that and make it slightly elongated, it seems to work fine...

17 hours ago, jbelsky said:

I just tried altering the "circular" oval NURB and it doesn't help.  Just makes the distortion more extreme.

There seems to be a problem with the way VW interacts with the "circular" NURBS object (note that it is in fact a different type of object and has a radius field in the OIP). I was able to get it to work by switching to top/plan, selecting and grouping the circular NURBS, scaling the Y direction up from 0.75" to 0.755" (use the centre point as reference to keep it aligned), ungrouping and then lofting.

Although there is a workaround I think its worth submitting this as a bug. The "circular" NURBS behaves differently with other operations as well. I normally troubleshoot loft issues like this by splitting the NURBS curves along their axis of symmetry but the Split Tool wouldn't work on the "circular" NURBS.

Kevin

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@zoomer, these shapes were all created as you described, by scaling the same elipse in order to maintain the correct direction and number of vertices, etc.

@Kevin McAllister, when I do what you've described (grouping circular nurbs, scaling in one direction, ungrouping), I get exactly the same result.

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1 hour ago, jbelsky said:

@Kevin McAllister, when I do what you've described (grouping circular nurbs, scaling in one direction, ungrouping), I get exactly the same result.

My apologies, I missed describing one step which I do automatically. I went into a side view and rotated the "circular" NURBS (Command L multiple times) so its directional arrow aligned with the others . I did this before scaling it. To turn on "show direction" click on one of the NURBS curves and select the "show direction" checkbox in the OIP.

Kevin

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1 hour ago, jbelsky said:

these shapes were all created as you described, by scaling the same elipse in order to maintain the correct direction and number of vertices, etc.

So I think more and more that this is just a Bug in VW (?)

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@Kevin McAllister Thank you! That does work.  As long as the arrows all seem to start in the same place, it lofts properly.

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