Hello!

The Shell is not working...Why? The prvious structure is OK but the second one is not OK. Can someony help me or check? Thanks.

Hi Andras,

It looks like a few of those shapes have a very small and very tightly bent twist in them at the top, here are some shots zooming in:










There is actually a little vertical edge in there, those pieces don't meet up in a single point like they seem to look like at first.

That kind of sharp sudden twist in the surface makes the surface normal shift suddenly by large amounts in direction in that small area of the surface, that kind of thing gives a lot of trouble for offsetting or shelling.

I think those surfaces (I think there are a couple like that) will need to be reconstructed to get a good shell there.


A few other notes on shelling:

MoI's shell function can have difficulty processing surface models made up of multiple pieces into solids unless they have a pretty simple structure, like only 90 degree planes or only surfaces that are all completely smoothly connected to one another.

Other than that shell can work better if you use it on a solid shape rather than on a surface model shape, or also to avoid problems you can try shelling each surface individually to get something to work with, instead of a trying to do a surface made up of several joined pieces.

There was some previous discussion of this problem here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=133.11


Also there have been several fixes to shelling for v2 so you'll want to make sure you are using the v2 beta instead of v1 for shelling stuff.

- Michael

Thanks ! :)

Here's another shell problem.

No problem shelling this surface (rotate polyline). Which is pretty smart since the surface kind of doubles back on itself


No problem shelling this surface (rotate freeform curve)


But this one, formed by moving two of the rim points along z, shell won't touch it!


What's the problem here?

Steve

Hi Steve, can you please post the .3dm file? It is hard for me to guess what might be wrong just by looking at images only.

But it appears that likely the problem is that your final shape there comes to a sharp point at the tip instead of being smoothly rounded at the tip.

That would make the offset of it not touch at a point anymore unless it was extended, which I don't think that the sheller is able to do in this case.

Typically for thickening a surface into a solid, the sheller tries to create lofted surfaces between the edges of the original and the offset, but in that case there is no edge on the collapsed pole point to match with the offset surface's edge, so that probably confuses it.

You can probably produce the shell manually in this case by using Offset instead.

- Michael

Hi Steve, also looking at that shape you've got a pretty tight bend around that brim area particularly in the last one - that will tend to limit the shell or offset thickness to a small value, it should be smaller than the radius of the tightest bend of the initial surface.

Otherwise the result would be a self-intersecting mess.

Here is a bit of illustration on this:



Notice how if we have a distance that is greater than the radius of the bend, it will produce bunching and self-intersection there as it tries to negotiate that bend? That is the kind of thing that can happen with offset and shell (and fillet too).

If you have a tight bend you will generally need to use a rather small offset distance or shell thickness as well.

- Michael

Thanks Michael,

The file contains the rotated curve and a copy which has been distorted slightly. It seems that as soon as the points are moved at all, even if the curve is not tight, shelling/offset won't happen.

Funny that, because when you halve the hat it will shell.
This one's for Michael, I think.

---------

Cool observation Danny, at least it provides a workaround. Although if you then mirror/copy the shelled half hat, MoI won't join the two halves!

Hi Steve,

>>...MoI won't join the two halves!

Delete the highlighted faces as shown, then mirror and apply the join.



Cheers

Cheers Danny. New tricks everyday:)

Hi Steve, thanks for posting the example. I'm not surprised that shell would have difficulty with that, but I would think that Offset should work which it doesn't.

Before you moved any control points around, that surface was a "surface of revolution" which is able to be shelled or offset as a special case, with an offset happening on the generator curve that is then revolved, rather than directly on the surface.

That kind of special case handling can help get better results in several situations, once you move some of the control points it is no longer handled as a special case because it is not a special surface of revolution primitive after that.

I'll save up this example and after this next beta is out I will take a look and see what is involved in getting this to work better, it would be good if at least Offset was able to produce a result as a first stage.

- Michael

Thanks, Michael for revealing more about the inner workings of MoI. It's amazing that the way offset works depends on the type of object in question.