Lofting Problem

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 From:  Brian (BWTR)
2791.1 
For some strange reason this will not loft.
Any advice/suggestions please?

Brian
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 From:  d^^b (DAVID)
2791.2 
There are a error in one of the frames. Two controls points are overlapping




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 From:  Jamie (FUTUREPROOF)
2791.3 
Hi Brian

was going to post the same thing. There are overlapping non joined points in the 2nd profile. Also it would be easier if there were less points on the curves. If you loft set to loose it will give a smoother result. Do you need to follow the profile exactly? you can adjust the points after you have completed the loft to tweak the shape.

I just made half to make it simpler.

hope it helps jamie















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 From:  Brian (BWTR)
2791.4 In reply to 2791.3 
Thanks. I could not see that.
It was made up of a mirrored shape and should not have shown up as a joined yellow anyway I feel.

Takes the gloss off using MoI with these silly things.

Brian
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 From:  Michael Gibson
2791.5 In reply to 2791.4 
Hi Brian, it's possible to mirror 2 halves and have the result only touch each other at one point and not at the other.

That's how you end up with a result like you had - here is an example, say I start with this curve:



Then if I mirror it, I get this result:




So notice there that those halves touch each other at one point, so they can be joined and will be glued there, making one large curve that will select as a single piece. But they are far enough apart at the other point that they will not be glued there and it will be an open curve.

That's the same situation that you had, but with the gap being smaller in size so less obvious.


> Takes the gloss off using MoI with these silly things.

Well, it's basically only doing exactly what you tell it in cases like this. It helps to be careful that pieces align properly before mirroring to avoid this kind of problem.

- Michael

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 From:  BurrMan
2791.6 In reply to 2791.5 
I think if you select that curve, MoI will tell you it's not a "Closed Curve". It will just say "Curve".
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 From:  Brian (BWTR)
2791.7 In reply to 2791.6 
Have you noted that the "faulty" one has no problem doing a loft with the one on the left!

See, in my logic, I tested all those pairs combinations and the faulty one tested OK that way!

How many people might want to use MoI on an occasional basis rather than as a fulltime, all day, every day, main app? (Where you remember all the quirks!)
This is where the greatness of MoI is let down by a raft of complexities counterproductive to it's working principle I think.

Brian
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 From:  Michael Gibson
2791.8 In reply to 2791.7 
Hi Brian,

> This is where the greatness of MoI is let down by a raft
> of complexities counterproductive to it's working
> principle I think.

Certainly in a perfect world it would read your mind and make what you had been thinking of and not what you actually drew.

But keep in mind that this is not a perfect world, and MoI does not have that kind of artificial intelligence - if you give it badly formed or messy curves as input then you will tend to get messy results or problems exactly like you have shown here.

I'd like to make MoI more tolerant of these kinds of things, but that is something that happens incrementally over time. It can be difficult to do too much "automatic fixing" of things because that can also alter shapes as well.


It tends to be best to take some care in forming your initial curves, use the snapping tools to make sure pieces are actually aligned with one another rather than just "eyeballing" it. If you make sure to use the various snap tools to get things lined up then you would not really run into this.

- Michael
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 From:  BurrMan
2791.9 In reply to 2791.7 
First of all, calling that curve "Faulty" is misleading to "yourself". That curve is exactly what you drew.

It does "NOT" do a proper loft with "ANY" of those curves. You'll notice if you try to loft it with that curve it just creates a "Surface". That surface is not a joined solid but has overlapping edges that are really of no value for what you are trying to do. You need to look at the results closer and what the application has already told you.

I would suspect that such a casual user would just expect that unintended results that he creates would be normal. Anybody wanting to use an application properly has to put the time in to learn how to do so.

Becoming involved with an application in it's creation state requires responsibility on the users part. If one doesnt have the time or energy to put into using an application, then that person should be aware that unintended results will be the "norm" for him.
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 From:  JPBWEB
2791.10 
Having sweated for years with Rhino first and then MoI trying to loft complex ship hull shapes and the sort, I have learned the hard way that it matters a lot to use the fewest, cleanest, simplest curves necessary to describe reasonably accurately the shape intended. It pays also not to insist on generating the whole shape at once if getting into trouble, but to go for parts that can be welded later (with continuity issues, my pet peeve, as a consequence). These days, I spend way more time optimising the curves than generating the surfaces and volumes, and it pays handsomely.

I do not get the impression that MoI is a let down in this matter, nor in any other for that. Simply I wish there would be tools to assess curvature and continuity, to rebuild curves, etc. But much of that is in Rhino that I always run in the background and is only one copy/paste away.
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 From:  JPBWEB
2791.11 
To illustrate my point, consider the following two curves.



The bottom one is the shape I want (based on a blueprint, etc.). I built it by blending two straight lines with a G2 blend, thus getting a 3-segment curve. If revolved, lofted etc., it will show identical segments in the surface, a less than desirable outcome if it can be avoided

I then drew a new single curve with control points in the same exact position as for the first curve, and then adjusted the point locations to match the original shape. I could not get a 100% match but in a matter of minutes, I could get a close enough equivalent.

I would love to be able to get MoI to do the fiddling with points for me and give me the single curve that matches as optimally as possible a multi-segment curve.
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 From:  Jamie (FUTUREPROOF)
2791.12 
Hi Brian

I fully understand your fustration. Nurbs modelling is not the easiest way to think and it is sometimes difficult to find out what is wrong. MoI is by far the easiest nurbs programme there is. You will find programmes like Rhino, form Z a lot more complex.

Below is a link to some free modelling software. A beta version but seems to work fine. It does not seem to be supported anymore (it was bought by dassault, solidworks catia people). I like it for quick ideas. very direct way of working. ultimately limited but can be fun.

http://www.cbmodelpro.com/about.html

A tutorial below.

http://s101.photobucket.com/albums/m47/TMeeks/CB%20Model%20Pro/?action=view¤t=SphereHelmet.flv

Anyway see what you think.

Jamie
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 From:  Frenchy Pilou (PILOU)
2791.13 In reply to 2791.12 
In same easy modeling aspect by Dassault and free (but not subdivision) take a look to
http://www.3dvia.com/software/3dvia-shape/ ;)
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 From:  DannyT (DANTAS)
2791.14 In reply to 2791.11 
Hi Jean-Paul,

RE: two curves

> The bottom one is the shape I want (based on a blueprint, etc.).
> I built it by blending two straight lines with a G2 blend,
> thus getting a 3-segment curve. If revolved, lofted etc.,
> it will show identical segments in the surface, a less
> than desirable outcome if it can be avoided

I have to disagree with your statement, the curve you created with the G2 blend will give you a technically smoother surface than the top 'single segment' curve.
The joins between those curves have no bearing on the smoothness, they are part of the NURBS world, if you were to create a surface, either by revolving, lofted etc. from the G2 curve and rendered it, there would be no signs of those joins, it would show a continuous smooth surface, and the 'single segment' curve would look good also, but it's not perfect.

I try to model with the simplest geometry possible, lines, circles and arcs where possible, in that way I know I have sound geometry, when I use the modeling tools on these curves things tend to work more smoothly, like Booleans and blends.

I reproduced your curves and took them into a CAD program that has analysis tools, (I think Rhino has the same tools) to show you what I mean.

Here are the curves in MoI, I added an additional one which is made up of 5 segments, 3 straight lines and 2 arcs.



Here are the three curves analysed showing there curvature combs, the smoother the comb the better.



As you can see the top curve is the most undesirable, technically, If you were asked to model something by a company who required A-class surfaces for that shape, sorry to say but the top 'single segment' curve would be rejected, if they analysed it that is.

If Rhino has the comb tool you can manipulate the 'single segment' curve until you get smooth looking combs.

Cheers
~Danny~

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 From:  JPBWEB
2791.15 In reply to 2791.14 
@ DannyT

I agree with what you say. It is a pity I discarded the curves I drew this morning, but I did evaluate the curvature with the combs tool that Rhino has too. I concur with your analysis, although I think that my “single” curve did not display such a poor continuity.

My point was rather that it would be better to have surfaces that are made with continuous curves (but proper, smooth ones) rather than segments because they lend themselves much better to further transformation like cutting, booleans and blends etc. than if there is a segment line in the middle of a cut-out opening for which you want a blend with another piece.

In this recreated example, the bottom curve is the 3-parts with G2 blend one, and the top is the single smooth one. The combs show similar fairness, although the G2 curve is somewhat cleaner. The surfaces generated with both curves look very similar, and indeed the various toolds in Rhino (zebra, environment map etc.) are nearly identical. The G2 curve is possibly a bit better, but not by much.



But I would anticipate less trouble with the surface generated from the single curve when working with the cut-out circles, as the edges running across a segment line are segmented.


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 From:  BurrMan
2791.16 In reply to 2791.15 
I think the segmented seam curves will become less of a factor as MoI matures.
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 From:  Michael Gibson
2791.17 In reply to 2791.14 
Hi Danny, just a quick note on continuity - when you have a curve that is a "single segment" (formed with only one single spline instead of multiple splines touching end to end), if it is the most common cubic curve than that does give you an internal G2 continuity throughout the whole spline unless there are special things like fully multiple knots or stacked control points.

That's for NURBS curves though - a "bezier curve" as often made by 2D illustration software is not as good for G2 continuity because it basically has fully multiple knots for every curve span.

It's pretty frequent for curvature combs to be somewhat misused and over obsessed with though - when the comb itself has cusp points in it, that is actually G2 continuity, that's when the curvature is equal at a juncture point.

When the comb is completely smooth throughout, that actually means G3 continuity, where the rate of change of the curvature is continuous, not just the curvature itself being continuous.

There is not really as much of a physically noticeable difference between G2 and G3 continuity like there is between G1 and G2 with reflection lines.

The other thing is that it can often be better to be more concerned with "fairness" rather than the actual technical continuity. Pieces can be technically continuous but still have poor curvature distribution. Continuity is about how things match up at a certain point but not really about how they are distributed which tends to be actually more important in shaping.

- Michael
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 From:  DannyT (DANTAS)
2791.18 In reply to 2791.15 
Hi Jean-Paul,

If you like no seam lines in your models, then yes, I agree in what your doing, but there would be some situations where you can't avoid them, I think the extra work has minimal effect for the final outcome, like the model with the seams is quite acceptable, you can still apply a fillet around the hole the only difference is you have to pick 3 edge segments, but it still fillets.

All I'm trying to say is one method is as good as the other, I don't think in saying that the model with seams, is less than desirable is a correct statement.

Cheers
~Danny~

EDITED: 21 Jul 2009 by DANTAS

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 From:  DannyT (DANTAS)
2791.19 In reply to 2791.17 
Hi Michael,

> It's pretty frequent for curvature combs
> to be somewhat misused and over obsessed
> with though - when the comb itself has cusp
> points in it, that is actually G2 continuity,
> that's when the curvature is equal at a juncture point.

I agree with this, I don't really use comb analysis much myself.
I was just trying to point out to Jean-Paul that there was nothing wrong with the curve with the G2 blend that produced seams in the model.

I was trained with all these rules in producing 'technically correct' curves and felt then that it took the gloss off 3d modeling a bit.
There's a definition I like for A-Class surfaces that I came across.

"An A-Class surface is a surface that is smooth and appealing to the eye"

So I figured no matter how the surface was built, as long as it looked smooth and flowing and the reflections were good, it didn't really matter what degree or how many segments and what the comb looked like for the underlying spline.

Cheers
~Danny~
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