loft surface: faulty tool?

[Bart Rammeloo]

Hi all,

I'm running into strage problems using the loft tool. The resulting surfaces don't always have the expected amount of CV's. Take a look at the following page:

lofted surfaces

This problem makes it very difficult to create clean models in VW, because you would have to start out with a flat surface in order to keep the amount of CV's down, and I fear that is not the way most people would work.

Anyone?

Thanks in advance and kind regards,

BaRa

[Biplab]

When you loft with general curve shapes, one of the things that is done (and I think this is true for other packages too e.g. Rhino) is to make all the curves compatible. What this means is that the curves are manipulated internally such that they all have the same number of "knots" at the same parameter value. This increases the number of knots and consequently the control points of the curves before they are submitted for actual lofting. Hence the increase in the number of control points. Making curves compatible provides a much better-shaped interpolated surface.

[Bart Rammeloo]

Thanks for the explanation, Biplab, but I'm afraid it's not completely correct. Try the following:

(1) draw a curve with the NURBS curve tool. Makes sure it's not a flat curve.

(2) duplicate it 2 times. By duplicating, you can be sure they have the same parametrization.

(3) connect it with the loft tool.

(4) the results is a surface with more CV's than you started out with, despite using 3 identical curves.

This the image before lofting, with identical curves and with a total amount of 12 CV's:

And this is the result after lofting, with a total amount of 18 CV's:

If I do the same thing in Rhino, Maya or StudioTools, I have no change in the amount of CV's. I know Maya and StudioTools have a different kernel than VW, but if I am not mistaken, Rhino uses SMlib.

So assuming Rhino uses the same NURBS-kernel as VW, why does it happen in VW?

Kind regards,

BaRA

P.S.: in Rhino, Maya and StudioTools you have the option to rebuild your curves, but you can also choose not to. Perhaps it would be handy to have that option included in the loft tool? It would mean a lot to people who want to change the shape of the surface after lofting. Or you could include history in the loft operation - in that case curve rebuilding wouldn't matter that much, because you could use the original curves to modify the surface. Either way, the current situation doesn't cut it, I'm afraid

[ 08-15-2003, 09:26 AM: Message edited by: BaRa ]

[Spencer]

I have been experiencing similar difficulties. I don't have anything specific to add now, but I wouldn't want BaRa's to be a lone voice.

The new tools are a great addition and much appreciated. Improvements are critical to my success.

Best regards,

Spencer

[Charlene]

Dear BaRA:

There are two modes when you draw NURBS curve: the first one is "NURBS curve by interpolation points" and the second one is "NURBS curve by control points". We calculate the control points internally if you choose the first mode, the number of control points will be bigger than the number of interpolation points. Try the following again, the only difference is that you draw NURBS curve by the second mode, you will get the result as you expect:

(1) draw a curve with the NURBS curve tool (the second mode). Makes sure it's not a flat curve.

(2) duplicate it 2 times. By duplicating, you can be sure they have the same parametrization.

(3) connect it with the loft tool.

(4) the results is a surface with the same CV's as you started out with.

>If I do the same thing in Rhino, Maya or >StudioTools, I have no change in the amount of >CV's. I know Maya and StudioTools have a >different kernel than VW, but if I am not >mistaken, Rhino uses SMlib.

>So assuming Rhino uses the same NURBS-kernel as >VW, why does it happen in VW?

I guess that you draw Rhino's curve by their first mode which is control points mode, but if you draw it by the second mode, you will get more control points than interpolation points. Let me know if you still have questions.

Regards,

Charlene

[Bart Rammeloo]

Charlene, you're my heroin I hereby humbly step back. I completely forgot that VW had those two options (knots and CV's). Problem solved, no need to say more.

Although it would be nice to have a rebuild option AND loft history.

On a side note: it think VW would benefit from a PowerPack only manual. Any plans there?

Kind regards,

BaRa

[ 08-15-2003, 11:29 AM: Message edited by: BaRa ]