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Help with Lofting tool?


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I'm trying to loft (with no rail mode) a series of complex closed NURBS curves that contain both sharp angles and smooth sections, making a container with flanges on the edge. The curves are all related shapes, but vary in size- a boat hull would be a rough analogy. The part is destined for injection molding.

It seems that to get smoothly contoured results I need to have the same number of vertexes functioning as alignment points on each curve in the series- otherwise the shape twists wildly. Am I correct in assuming that the vertexes work as alignment points only if they are interpolation points, rather than control points? When I select the curves with the loft tool and step through the series of vertexes on each curve in sequence, the alignment guide "skips over" some points that seem to me to be interpolation points. I have tried turning them into points that would be used as alignment points by using the "convert points" option on the 3D reshape tool, but the results are unpredictable. As close as I can look at them the points appear to be on the NURBS curve, but the alignment guide (the little red line) in the loft tool won't snap to them. This seems as if it might be related to an earlier problem I had (I believe it was confirmed as a bug ) in which vertexes would "drift off" the NURBS curve and could not be brought back into alignment.

Am I using the right approach? Is there a way to know the type of point I am dealing with, or is that not what determines the alignment of the lofted shape? Do the degree settings have an influence I should be exploiting? These shapes started as 2D polylines, and were converted to NURBS- would I possibly do better drawing them as NURBS from the start?

Any suggestions for using the lofting tool would be appreciated.

Thanks, Ron R

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As soon as the nurbs curves become complex, lofting them can be extremely frustrating. I have had the same problems and found no ideal solution. I think you should consider doing the flanges and the smooth parts in separate operations. Or - if it is possible to mirror the object along some axis - cut the shapes in half. That way you will get more reliable alignment points.

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Breaking the curves up into smaller segments was going to be something to try. I have tried without the flanges, and they actually seem to aid in the alignment, as the guide always finds the vertex if it's a corner. If I can get the same number of vertexes in he right positions to be found by the point alignment guide in the lofting tool, the loft works beautifully.

Can any VW gurus tell me how the tool choses the vertexes it uses for point alignment? I feel if I can understand this I will be able to solve the problem.


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Without getting into real nitty gritty math, I can tell you that using nurbs curve from the beginning will most certainly increase your chances for success; however, you mention that you have sharp angles...Without seeing your shape I can't say for sure, but whenever I have "breaks" or angles between surfaces, I model them seperately and join them later if possible.

Good nurbs modeling requires knowledge and some due diligence in setting up the surfaces. You are approaching your surface creation properly by getting the same number of verticies in proper alignment for the lofting tool. The verticies you're thinking of for point alignment that VW uses are generally the control points, so having the same number of control points on each curve in the loft is the best way to create a surface...and alignment (not necessariliy straight) of the control points in each curve will also help ensure success. A nurbs surface is defined by several factors, but the control points are the independent or definable values used to construct the curve. Try these links below to get a little more insight into nurbs....the first link particularly, though not directly addressing control points give some insight into nurbs surfaces. Constructing a good network of curves to loft a complex surface is definitely tedious and where all the work should go.



[ 12-02-2004, 10:23 AM: Message edited by: tom kyler ]

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