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 From:  ppj
3575.1 
Hi,

I can not do the sweep properly, if you could provide some help, thank you

ppj



start:



bad matches:

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 From:  Michael Gibson
3575.2 In reply to 3575.1 
Hi ppj, the corner mitering mechanism may have difficulty on paths that are swooping around in 3D rather than all in the same plane.

Mitering is done by an initial sweep along the main part of the segment, and then extensions of the surfaces.

When your path curve swoops and bends around all over the place in 3D instead of only in 1 plane, it is likely that the extended portions do not have a very clean collision between them anymore.

You will likely get the same kind of problem elsewhere, for example here I have opened your file in Rhino and tried the Sweep1 command in there:



As you can see it gets similar results.

Primarily for a mitered corner sweep you would want either a path all in a single plane, or something made up only of line segments, not something that has bendy parts and also not planar.

You may want to put in some rounded corners (trimming back and using Blend is one way to do this) so that you have one smooth rail instead of a segmented one like that.

- Michael
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 From:  Michael Gibson
3575.3 In reply to 3575.1 
Hi ppj, so the other kind of general intersection problem with the kind of thing you have here is the very shallow angle between pieces like this:



With those surfaces being nearly coincident, it makes it difficult for an intersection to be clearly identified between them, there is too much of the surfaces that are potentially part of the intersection instead of a distinct line where they collide.

- Michael

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 From:  Michael Gibson
3575.4 In reply to 3575.1 
Hi ppj, here is a kind of more generalized example to try and explain why swoopy bendy non-planar paths and mitered corners do not really go together so easily.

Here I've made an shape that has a more pronounced non-planarity to it so you'll be able to see the kind of result a bit easier:



Building a sweep from that:




If you zoom in on the juncture area a bit, you can see that swoopy paths just do not produce the same kind of uniform miter cut where they touch one another like you would get on a planar path:



In fact in this case the intersection curves between the surfaces look like this:




The general problem is that a nice mitered cut corner needs to have a lot of regularity throughout the area where the pieces are touching one another.

When you have a path that starts to bank and curve around the instant that it leaves the corner, it means that there is not that regularity other than in the tiny area right at the corner.


Another thing you could try to get something more like what you want would be to try and get some line segments in there for the corner areas where pieces are touching, when a sweep follows a line segment in the intersecting area as it leaves a corner, that provides a lot of regularity and matching between the pieces....


Anyway, your shape is not as non-planar as the one I show here, but it is still nevertheless nonplanar and so you will be getting the similar kind of effect as this case, just to a lesser degree that is harder to see.

- Michael

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 From:  Michael Gibson
3575.5 In reply to 3575.1 
And here's a more close up example from your own shape.

If we examine just these 2 surfaces here:





If we get in there closely and examine them with a high resolution mesh, you can see that they are not actually intersecting in a clean way, their edges do not actually cross over one another, one's edge is somewhat to the inside of the other:




That's because at the part where they intersect, the sweeps are bending away from each other following different curved 3D paths instead of staying on the same level as each other as they would in a planar path.

- Michael

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 From:  Michael Gibson
3575.6 In reply to 3575.5 
Another way to describe the situation in that last post above is that at the area where it looks like they are coming together here:




The shape of each surface actually comes from an internal part of the path.


Like this surface here at that point comes from this part of the rail:




And the other surface's shape at that point comes from this part of the rail on the other side of the corner:




And because those points on either side of the corner are not planar to one another but instead are kind of arcing and bending around in 3D (once they leave the shared corner spot), it means that there is not an even alignment between those pieces where it looks like they would exactly touch. I mean they do touch but not in a complete full intersection that divides each one into 2 distinct halves.

That's kind of fundamentally why mitering can't really work on things like this.

- Michael

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 From:  Anis
3575.7 In reply to 3575.6 
Also the profile too big for the path
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 From:  Anis
3575.8 In reply to 3575.7 
@Danny,

Can you please try this case in NX ?
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 From:  ppj
3575.9 
Hi Michael,

Many thanks for your help ; I really appreciate you took the time to answer my question . I have understood you explanation. I think I will blend the Extrude/Bloolean/Scale functions. I am really statisfied by the boolean function. I will let you know if I get any results.


Thanks,

Philippe
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 From:  DannyT (DANTAS)
3575.10 In reply to 3575.8 
Hi Anis,

> Can you please try this case in NX ?

NX handles this OK but we're talking about two different geometry engines here, NX is using the more mature, robust and expensive parasolid kernel.
I think Michaels comparison there with 'like' programs is a better comparison, apples with apples, if your trying to compare with NX it's like comparing apples with an apple juicer.

Cheers
~Danny~
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 From:  Michael Gibson
3575.11 In reply to 3575.10 
Hi Danny, actually although the NX generated one looks great at a first glance it actually has some problems in it.

It seems to be constructed with a different technique of kind of sliding out the control points for the ends of sweep segments.

But that technique has produced some areas where the control points have been slid backwards so that the created surface is a badly formed self-intersecting one.


Here's a zoom in to one problem area:









Note that the surface in that spot has a sudden abrupt 180 degree hook in it:




If you turn on surface control points you can see what has happened, there were several straight sections coming up to the miter, and then it produced the miter by shifting control points around, but that caused the end of the surface to come partly back over one of the previous straight sections:




The other short segment has the same kind of problem.

That particular technique that is being used there works pretty good if the sweep path is not very tightly bent in that zone near the miter (because tightly bent ones will have numerous control points all over it), and if the miter angles are smaller so that they don't shift the points back over a previous section so much.


There is another possible technique which is to kind of place in slanted profile curves at the corners of each miter spot and use those as profiles for the sweep. Rhino actually seems to have a somewhat new option for that called "untrimmed miters" which works pretty good for this case but does not work well for more bendy paths.

That would probably be a good option to have in Sweep in MoI too though. But that kind of produces a different kind of problem where creating a surface between angled profiles makes for some variation in the "tube thickness" because the sweep profile has a kind of angular turn to it as it moves along the path, similar to the example in this previous post:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3546.6


So anyway as you can see it is not really an easy problem to solve, even NX / Parasolid did not get it right... and I think that may be due to it actually being a geometric impossibility to have exact constant tube thickness and also exactly meeting miters on non-planar curved segments like this.

- Michael

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 From:  ppj
3575.12 
Thank you very much, very impressive NX. I thought exactly this :-)

Philippe.

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 From:  ppj
3575.13 
Well done Michael.

Export is deformed following the default. Clearly.

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 From:  Michael Gibson
3575.14 In reply to 3575.13 
Hi ppj, for rendering purposes that will probably be ok though since the messy area is small in size and also kind of internal to the overall shape.

But that kind of self-intersecting surface will cause problems in many other kinds of operations, things like booleans will have problems operating in those areas because the intersection calculation between something like that is not very well defined. So from a kind of "NURBS solid model integrity" standpoint it is not good to have results like that.


Probably in v3 I will be able to experiment with an alternate mitering technique of basically taking the profile curve and rotating and stretching it and placing it at the corner areas, which should give a non-intersecting result usually but will probably not be 100% even tube thickness since it will make a kind of slanting effect.


- Michael
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 From:  Michael Gibson
3575.15 In reply to 3575.10 
Hi Danny, it would be interesting if you could try the attached sweep example in NX and see what it does to it.

This one has a higher degree of non-planarity and bendy-ness to it, it should show how swoopy things are not really guaranteed to actually meet at a common normal miter.

I wonder if they will do something like twist the surfaces at the very end to force them to align or what...





- Michael

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 From:  DannyT (DANTAS)
3575.16 In reply to 3575.15 
Hi Michael,

I knew you'd jump on this as soon as I posted it :) I was on my way out and was waiting for the wife to put on her finishing touches and didn't have time to analyse the sweep, I was going to try the tuber_miter as well but the wife had finished and we had to go :(

So I'm back now and posted the NX version of tuber_miter, and again no time to analyse it, but I'm sure you'll tear it apart Michael ;)

Cheers
~Danny~
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 From:  Michael Gibson
3575.17 In reply to 3575.16 
Hi Danny, thanks for giving that one a try!

It's an interesting result - it looks like it is forcing the miter join by warping the ends of the sweep, I guess maybe in the same kind of tweaking of the control points at the end, although maybe it is a bit different than the other one.

That produces a kind of miter-join result but at the expense of not having fully regular thickness tube, just by eyeballing it you can see the thickness is different in these areas for example:





Here you can see why there is not a constant thickness - here I've set up a bunch of isoparm curves on one of the other surfaces and as you can see the warping at the end causes a kind of slanting to the cross-sections in the generated surface, they are not perpendicular to the central rail anymore:




However, that does not seem to really be a particularly bad result, because I'm pretty sure that it is a physical impossibility to maintain constant thickness in this situation and also have a nice matching miter, you can have either one or the other. And if you have bothered to have a single joined rail curve with a kink in it, I guess that probably means you are actually more interested in the miter.


- Michael

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 From:  Michael Gibson
3575.18 In reply to 3575.16 
One thing I'm not quite sure what to make of with that NX result is that the shape at the miter here:



Has actually had a kind of shearing happen on it, if you lay that flat down on a plane it looks like this:




I guess it's actually a stretch of the profile which is something that goes along with a miter corner, but I'm not sure why it stretched in that particular direction.

- Michael

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 From:  Anis
3575.19 In reply to 3575.10 
Hi Danny,

I am aware about the geometry kernel.
I want to know the result in NX, is NX capable or not ?

Thanks Danny :)
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 From:  Anis
3575.20 In reply to 3575.19 
Hi Michael and Danny.....

I tried this case in SWX ( use the same kernel as NX ).
For case #1 = SWX cant produce any result. I dont know, it is the software or I dont understand the correct technique to get the result.
For case #2 = see attached

Hope this will give you more data to examine.

Thanks
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