Michael why I can not get the same amount of points on surface as I have on curves? by doing loft (loose, exact)

Hi Andrei,

> Michael why I can not get the same amount of points on surface
> as I have on curves? by doing loft (loose, exact)

It's because your curves are closed and do not all have a uniform start/end point to them. Inside of Loft in order to connect closed curves up without a twisting action happening to them, it has to basically cut the curve into pieces and rejoin those pieces so that the curves can connect together without any twisting happening between them.

So after the curve seams are aligned each of the curves ends up with a slightly different structure in them, and in order to do a loft the structure of all the curves get combined together into the generated surface.

Normally you should not really worry so much about how many control points is in the generated surface, look at Loft as more about generating a piece of material from the curves that follows their shapes. If you need to edit the loft it's better to edit your original curves rather than trying to do control point editing of the generated surface itself.

- Michael

Michael sorry but could you pleas explain it with screens or in another words, thank you.

Hi Andrei - every closed curve still has a start and end point to it. When you do a loft between closed curves, if it just connected the curves together by this natural start points it would often product a twisted result unless those points were all aligned with one another.

So in order to reduce twisting MoI goes through a process of "seam alignment", where it relocates the seam point of a closed curve by cutting the curve at the new seam point and gluing together the other pieces. This seam alignment step changes the structure of the curve so the "seam aligned" curves are not exactly the same curves as your starting ones. That's the reason why they don't have the same structure when they are lofted.

- Michael

Yes I notice it, only good results I could get was from copying first curve and them modify it. But when I draw second curve I got that twisted result. I need it to have ability to modify surface by moving points after it was trimmed. That is why I wanted to have predicted amount of points. Is it any way to align second curve that was drawn to get not twisted result?

And could you pleas tell me the logic that used to create seam. As I can see it is not lay where curves was closed.

Sometimes twist can be avoid by select a curve and type "flip" after pressing the tab key!

what this comand do?

Flip: Curve don't move, stay in place but its "way" is inversed! Start becomes end, and end becomes start!

It's the same when you draw a curve from left to right or right to left : it's way is inversed!

Here with the function Linweb, so the result is very different!
Lineweb : http://moi3d.com/forum/index.php?webtag=MOI&msg=3666.1

Hi Andrei,

> But when I draw second curve I got that twisted result.

What twisted result is that? Sorry but I'm not understanding this part. What kind of twisting are you seeing?


> I need it to have ability to modify surface by moving points after it was trimmed.

Usually you will need to delete the surface, re-loft it and retrim it for this to happen.



> Is it any way to align second curve that was drawn to get not twisted result?

There isn't really any way to do this. If you want to do a lot of surface control point editing, probably a sub-d modeling package is a better fit for that kind of workflow.



> And could you pleas tell me the logic that used to create seam. As I can
> see it is not lay where curves was closed.

Right, it does not use the natural closing point of the curve because that would often generate a twisted result. Like in your case here the natural closing point of each curve are on these spots here (points created by MarkCurveStart command from http://moi3d.com/forum/index.php?webtag=MOI&msg=5288.21):



See how the start point of each of those curves is in a totally different location? One on the top, the next one on the bottom, other ones on one side - if Loft tried to just directly connect those curves together into a surface the surface would be really messed up with a severely twisted and knotted up type form to it.

So when closed curves are lofted there is a twist minimization stage done which determines a better seam spot on each curve. This analysis works by connecting together pairs of profiles with line segments between them and looking at the total length of the line segments, it then looks at potential seam locations and narrows down to the location that reduces the length of the connecting line segments. The very first curve will have its seam stay in its natural location but all the other ones after that will have adjusted seam spots in order to minimize twist in the connection.

- Michael

Hi Andrei, or what you might want to try for some better surface editability is to set the "# Points" (number of points) mode for the Profiles option in the loft.

That will do a rebuild of each re-seamed curve using the number of points that you specify, and because the rebuild happens after the curve seam changing, the lofted surface result with that option will have a simple control point structure to it.

You can set the "Point count" option that shows up underneath the "Profiles" option to adjust the density of the generated surface.

If you want to do direct control point editing of the generated loft surface that would probably be your best bet.

- Michael

Pillow, thanx, good script)
Michael,
>What twisted result is that? Sorry but I'm not understanding this part. What kind of twisting are you seeing?
I mean that surface do not get exact amount of points that was on curves.

Everything other is clear) and MarkCurveStart command is good, thanx)

I know about "# Points" good option, but sometimes points appear not on the places where I wanted it to be.
So I resolve this problem very simplу I just copy first curve and modify it)

Hi Andrei,

> I mean that surface do not get exact amount of points that was on curves.

Ok, sorry - I thought you meant some kind of twisting in geometric shape.

- Michael

Hi Andrei,

> I know about "# Points" good option, but sometimes points appear not on the places
> where I wanted it to be. <...>

The next v3 beta will be able to refine a local row or column of a surface to have more control points in it using the Edit > Add pt command, it may help out for this kind of stuff.

- Michael

another words it will allow to add points to surface, like rhino?

Hi Andrei, yes it's like the InsertKnot command in Rhino.

- Michael

kool, and delete also?

also where i can read about this knots I hear about it often when watch rhino tutorials, and kinks if im not wrong.
And also in rhino normal direction means, but in moi i never meat problem with this, what is the answer)?

Hi Andrei,

> kool, and delete also?

Nope, no delete yet.

- Michael

Hi Andrei,

> also where i can read about this knots I hear about it often when watch rhino
> tutorials, and kinks if im not wrong.
> And also in rhino normal direction means, but in moi i never meat problem
> with this, what is the answer)?

MoI is designed to try and minimize the need to understand every single technical detail of NURBS modeling.

"Knots" are one of the pieces of information that make up NURBS curves and surfaces - every surface is made up of a set of points, weights, a degree value, and a knot vector, not just only a set of points alone.

The knot vector tends to be one of the more difficult pieces to describe very well - it's related to the number of "spans" of the curve or surface. Each span is sort of like an internal segment of the curve and the knots are where the spans touch each other.

Check out here if you want an in depth technical description of all the details of NURBS mathematics:

http://www.mactech.com/articles/develop/issue_25/schneider.html
http://web.cs.wpi.edu/~matt/courses/cs563/talks/nurbs.html
http://www.rhino3d.com/nurbs

- Michael