Hi Pilou, if I understand you correctly, you will want to extract these 2 curves (select them and then use Copy + Paste):



Then go to the Front view and use the edit frame to flatten them down to be planar, like this:



Now go to the Top view and use Construct / Offset to generate an offset curve:




That offset curve will become the new outline - you will then need to use Edit/Separate on the current solids to break them into surfaces, delete the side walls, untrim the surface, retrim one of the top or bottom surfaces with the offset curve, and then use Shell to thicken that surface back to a solid again.

- Michael

I will try that!
I made a sweep 2 rails but the angular part bottom and sections were not easy to make ;)
And surely false :)



Ps In your system the second curve bottom seems must have the same process!
In your image I see only one? Or they are very close?

Hi Pilou,

> In your image I see only one? Or they are very close?

It looked like they were mirrored copies... So either mirror or if they are not identical then repeat the same process on the other side.

If after untrimming the surface is not large enough to cover the new expanded trim outline, you will need to extend the surface somehow, possibly by scaling or stretching control points.

- Michael

< Mirrored
No I speak about the "under" other curve ;)

< That offset curve will become the new outline
Yes but there is another one "under" !

Hi Pilou,

> In your image I see only one? Or they are very close?

Ok, I see what you mean - I would just throw one of those and then after you construct the new surface use Shell to thicken it into a solid, rather than repeating the steps again although that would be work as well.

But it looks like maybe you are saying that the original object is not a proper shell to begin with? If so then yes you would need to repeat the steps for each surface then instead of using shell.

- Michael

Hi Pilou,

> Yes but there is another one "under" !

Easiest is to construct that one by Shell or Offset of the top one after it is finished if that is possible.

If the surfaces are not actually offsets from one another, then it would seem strange as to why you would want to preserve their current thicknes....

- Michael

< But it looks like maybe you are saying that the original object is not a proper shell to begin with?
Yes that was the problem : offset & shelling works not well because sides are inclined and curvated! :)

Anyway you can see the basic idea how you can extract the generator curves by duplicating the edges and then flattening them.

Then you have some 2D curves that you can work on again with stuff like offsetting them.

Of course it is easier if the original generator curves were preserved, then you would not have to do that kind of extraction.

- Michael

< the basic idea how you can extract the generator curves by duplicating the edges and then flattening them.
That's the trick!
Does it possible to imagine that as a script of a commun function? As an "extract generator" or it's trivial as just the box frame can do the job?
But I must make the modification now for see if all that works :)
Because the flat curves must will be curvated again!

Hi Pilou,

> Does it possible to imagine that as a script of a commun
> function? As an "extract generator" or it's trivial as just
> the box frame can do the job?

It's hard to make an automatic function because it is not always clear to the algorithm which direction the 3D curves should be flattened to.

The flattening happens quite quickly anyway with the frame edit.

One other thing to note - this model is kind of messy, it looks like it was not mirrored with very good accuracy, there are parts that overlap.

For example in the Right side view:




If you zoom in a bit you can see the overlap:



The pieces do not mate exactly and they are not centered around the origin, they kind of lean to one side and each piece sticks through the other instead of touching the other exactly.

So that's going to cause a variety of problems such as surfaces not joining, etc..., so probably the model needs to be tuned up to be more accurate to get better results.

- Michael

Hi Pilou,

> Because the flat curves must will be curvated again!

No, they don't have to be manually curved - they stay planar and then are used to trim the surface.

That will generate a curved edge on the surface where it is intersected by the planar trim curve projection. That's all built into Trim, you just cut the surface (after untrimming the surface to prepare it for new trimming) with the planar curves and that's done.

- Michael

Hi Pilou,

Attached here is a cleaned up version that has that messy overlap area fixed and simplified, this one will shell.

- Michael

Ah damned!
It's for that something was curious when I Offseted the curvated curves there were a little difference!
Straight horizontal bottom will be not horizontal after the mirror!

So with your surface the shelling works but there is a little problem :)
Shelling is "vertical" 90° to the normal !
Or the original bottom edge is inclined ;)

Hi Pilou,

> Shelling is "vertical" 90° !
> Or the original edge is inclined ;)

Yeah, if you want a constant thickness throughout a piece, that's how it works!

The original piece also has pieces that don't line up with one another, do you want to keep that as well? :)

You could also do an Extrude of that surface that I posted previously instead of a Shell, does that give more of the desired result? It looks like that may be more of what was intended here since one side is nearly straight up vertical.

- Michael

So for example with Extrude you get this kind of result:





That seems to be the kind of edge arrangement that you might want.

But just be aware that extrusion of a curved object in one set direction does not actually create something that has a constant thickness throughout the piece.

Here's an example where you can see this more clearly - here I have a curve and then a second copy of it a small distance vertically above it. This is like extrusion, and you can see that the thickness is not equal in all areas:



When there is less curvature it becomes more difficult to see it clearly though.


It's only shelling or offsetting that creates things with constant thickness everywhere, unless the thing being extruded is planar in which case extrude is equivalent to shell.

- Michael

Yes :)

I have now the original good half-surf that must be "extand" by 2 units
Edge Bow and poop are inclined!

I try to extract and offset the curvated curves but that is not always success!

Hi Pilou,

> I try to extract and offset the curvated curves but that is not always success!

For which step do you not have success? The copying, the flattening, the offsetting?

I just tried doing it on your model over here and did not see any problems...

If you have a problem with an offset for example, could you post a model file with the particular curve that you are trying to offset?

Also I would still recommend to delete everything instead of one surface and work from there, then extrude the final result to get the solid.

- Michael

you can't extrude because bow and poop are inversed inclined!

Hi Pilou, attached here is a finished result.

> you can't extrude because bow and poop are inversed inclined!

Then simply do an extrusion which creates the proper edge direction in the back, and slice the result with a line in the front to cut it to the desired angle.

The attached result is an extrusion so you may need to cut it with a line (since it is a solid use the boolean difference for cutting it) to get the proper angle. But other than that it seems to be all set up.

The untrimmed original surface was not quite large enough to cover the offset curves completely, so I extended it by adding a tangent line to either side and extruding it.

- Michael

Hi Pilou, also of course if you wish to make adjustments, it is easier to preserve the original curves, either hidden or saved to a different file or something. That then eliminates the need to extract them to recreate them.

Also it's good to know how the original was created, like is it a shell, is it an extrusion with a piece cut off of it, and so forth...

All that information from the original creation makes it easier to reconstruct the model later.

- Michael