Shell not working

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 From:  rhodesy
2780.1 
I've just been trying to create a quick chair design and I have made the seat/back shape and now wish to shell it. Unfortunately nothing happens when i run the command. The chair isn't to any specific scale just now but I know what sort of range i should be scaling it by by making measurements within the scene. There doesnt seem to be anything too demanding with the shape and there aren't any tight creases so im a bit stumped - i thought shell had been tuned up relatively recently. Using June23 beta.

Cheers for any help here,
Rob
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 From:  manz
2780.2 In reply to 2780.1 
Hi Rob,

There is a problem at the top of the surface, the center spline is outside the surface boundary. Making an offset shows unwanted geometry.



Zooming in to that area on your surface shows:-




I made an edit by splitting the side splines and making blends between them, and removed the cp point that was outside the boundary. Then made a net surface:-




Shelling is now possible.




- Steve

EDITED: 3 Aug 2009 by MANZ

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 From:  Michael Gibson
2780.3 In reply to 2780.1 
Hi Rob, I think Steve's got you covered there - if you zoom in to that top area really closely like he shows there is a small area of weirdness right in that "pole" area where the surface kind of folds back over itself.

That kind of a thing can tend to happen when you've got a lot of shapes converging together and trying to collapse down to a single "pole" point.

Once you've got a micro fold in a surface, offsetting or shelling will become problematic because they try to make a surface that follows the surface normal, and the surface normal flips around very wildly and kind of violently in that little area.

When you do run into a problem with Shell, it can help to try Offset like Steve did there, because it will tend to generate the surface even though it is messed up and you can see what spot is messed up.

It tends to be more Offset/Shell friendly to try and construct things without poles when it is possible - often times you can instead build a larger more rectangular kind of sheet and then trim it to make the final outer boundary, rather than trying to construct a surface that hugs it more directly (which tends to cause bunching/compression in the surface in cases like this).

So for example here you could take a curve like this:



Sweep it to make a rectangular base surface:



Then Trim that surface with your other bounding curves:





Something created in that way where the outer boundary is a trim curve rather than the underlying surface itself trying to be stretched and compressed can be handled a lot better by a variety of further operations, especially Offset/Shell.

- Michael

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 From:  rhodesy
2780.4 
Thanks guys and so sorry for the late reply in this, i've been away. I see the extra point at the top of the middle spline there that has crept in so that makes sense of that mystery thanks. It is disapointing though that the shape cant be achieved just using those three splines, when i try and do it moi gets itself confused and brings in an exagerated twisting in the middle but the rest of the shape is ok. I guess its a bit of an awkward shape but annoying all the same as I would have thought this type of work would be where moi/nurbs would come into its own without resorting to work arounds.




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 From:  BurrMan
2780.5 In reply to 2780.4 
Thats funny. As soon as I fixed your geometry I lofted the surface with those 3 curves.

EDITED: 19 Jun 2012 by BURRMAN

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 From:  Michael Gibson
2780.6 In reply to 2780.4 
Hi Rob,

> I guess its a bit of an awkward shape but annoying all the same
> as I would have thought this type of work would be where moi/nurbs
> would come into its own without resorting to work arounds.

It's not an uncommon problem when you are trying to have multiple different shapes kind of "forced" to collapse together into a single point.

Things will tend to kind of stretch at different rates and kind of twist...

If you have things twisting and collapsing, that just does not tend to make good geometry.

Here's a screenshot of the control points for your original shape:



You can kind of get a sense there where different shapes (the sides versus the less curved spine) are all trying to come together but at the sides are trying to cover a larger distance and rushing together kind of faster, making bunched points, etc...

> would be where moi/nurbs would come into its own
> without resorting to work arounds.

It just does not physically or geometrically work very well to try and force different shapes to suddenly touch one another.

It's kind of similar to if you were to try and build that thing out of 3 equal lengths of wire - the curved ones follow a much longer perimeter than the central one, so there is not really anywhere to put the extra length of the central one, it would get all bunched up.

When you're using a surfacing command like sweep or network, things are connected together between profiles in a more equal way similar to that, when the shapes are far from equal that will be a bad matchup.

Remember NURBS are not magic - they work within a certain framework especially individual surfacing commands.

- Michael
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 From:  rhodesy
2780.7 
@Burr, aye the single sided loft works fine but its the shell not the loft that is the issue in this case.

@Michael, yeah I see there the problem and initially wanted to use network for this shape but that wouldn't work with just the three splines the way they were arranged. So I did split the lines (like manz suggested, (thanks)) and used a blend to create extra lines so network would work and that makes for nearly perfect results - just some funny nicks at some junctions which can be sorted out in post if needs be.

Cheers

EDITED: 23 Jul 2009 by RHODESY

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 From:  BurrMan
2780.8 In reply to 2780.7 
>>>>@Burr, aye the single sided loft works fine but its the shell not the loft that is the issue in this case.


Sorry, tunnel vision on reading the entire thread. My bad!
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