Hi Jean-Paul, I was messing around with your wireframe a bit more and I thought I'd share one note that may be generally useful to you when trying to use this kind of "make skin over wireframe" type method.

It's regarding this area where you may have a 3-sided region you want to fill in:




You've probably seen before where doing a Network on those 3 sided areas separately will make some undesirably bulging/puckering like this:



That happens because the curve pieces are of different lengths, and the upright profile can kind of sway back and forth as it gets merged with the other pieces, making for a kind of twist in shape.

But you can use Sweep with a special option enabled as an alternative which will work better in this situation, if you've got a simple planar profile piece like this:



What you do is select that planar profile, then run Construct / Sweep, and activate the "Maintain tangent" option:



That will cause the sweep to get generated by keeping that profile in the same plane and marching it down the rails and getting intersections with the rails and the plane, if you look at the control points of it you can see that regular planarity to them:



(EDIT: you would actually have seen it even more clearly if I had taken a top view snapshot, here to see that the surface control points are all arranged on parallel planes).

That's different than a regular sweep where the profiles kind of "slide" along the rails and can get that kind of twisting introduced if they are of different lengths.

Sweeps that are done with the "maintain tangent" option will more rigidly preserve the shape of the initial profile, which gets this kind of a result even when doing them as 4 separate sweeps:



- Michael

Michael,
Thats nice! One thing I was doing the other day when using network, was utilzing "RebuildCurve" to make all the curves have the same amount of points. I thought it was making a difference in the result! ???

Was I correct and does this relate to the different length thing you just pointed out?

Hi Burr, well it is hard to say anything for sure without seeing the particular curves...

But Network actually does a process like RebuildCurve internally. However, instead of doing it to a fixed number of points, it continues to add points until the rebuilt curve achieves a maximum distance tolerance away from the original input curves.

So if you smoothed out the curve or made it less wiggly or something by doing a RebuildCurve on it, that could possibly make the difference that you saw.


> Was I correct and does this relate to the different length thing you just pointed out?

Well, rebuilding several curves doesn't necessarily make them the same lengths even if they have the same number of points in them.

Maybe focusing on the length was not the right way to describe it - basically when you do a Network there is a kind of averaging process where the result is a combination of all the input curves in a equal way.

But in the kind of situation that I was showing previously, it is not really good for the input curves to be treated totally equally, it's better in that case for that upright profile curve to kind of have more emphasis placed on it kind of making it more of the primary shaping agent rather than a more equal blending between all the curves.

- Michael

Michael,

Thanks for your quality input. This thread has made me learn a few more tricks. I never quite understood before what the “Maintain Tangent” option did. I had tried tu use it but I saw no noticeable difference. Now I know (at least I think I know…)

On a related topic, what about this cage editing tool that is part of Rhino 4.0 ? Is this a kind of thing that would make smooth deformations easier for NURBS objects? Is it on the wish list for MoI ?

Hi Jean-Paul,

> I never quite understood before what the “Maintain Tangent”
> option did. I had tried tu use it but I saw no noticeable
> difference. Now I know (at least I think I know…)

Yeah it won't have an effect in just any sweep, the conditions need to be right for it to be able to be used, like the profile needs to be planar and if there is more than one profile they have to all share a same tangent direction where they touch the rails.

Also if you have exactly symmetrical rails then it would not really do anything because in that case the profile also slides along each rail in equal distances with each step.

It's when the 2 rails are different in shape and length that the profile will tend to twist as it is moved by sliding along the rails since the "slide distance" on each side is different in that case, except not with "Maintain tangent".

Maybe it should have been called "parallel planes" instead.


> Is this a kind of thing that would make smooth deformations
> easier for NURBS objects? Is it on the wish list for MoI ?

Yes, it would help to kind of squish objects around. It is on the wish list for MoI but I'm not sure when it will happen. It is difficult to make it work on a solid that has joined edges and still keep the edges stay connected in the deformed result, especially when the joined edges are trim edges that are located somewhere internal to the underlying surface rather than a natural surface edge.

- Michael