GROUP vs UNION

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GROUP vs UNION

stonysmith
I saw some discussion earlier, but was there ever any work done to allow
exporting separate, overlapping shells to a STL?

group(){
cube([1,1,1],center=true);
sphere(r=.75,center=true,$fn=16);
}

such that the stl would contain the two separate objects, NOT welded
together thru CSG?


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Re: GROUP vs UNION

kintel
Administrator
On Feb 8, 2012, at 00:45 AM, Stony Smith wrote:

> I saw some discussion earlier, but was there ever any work done to allow
> exporting separate, overlapping shells to a STL?
>
This is not (yet) supported, but would be interesting to look into.

 -Marius


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Re: GROUP vs UNION

Gordon Wrigley
It's a bit odd, what would happen if you then nested that inside another CSG operation?

On Wed, Feb 8, 2012 at 10:54 AM, Marius Kintel <[hidden email]> wrote:
On Feb 8, 2012, at 00:45 AM, Stony Smith wrote:

> I saw some discussion earlier, but was there ever any work done to allow
> exporting separate, overlapping shells to a STL?
>
This is not (yet) supported, but would be interesting to look into.

 -Marius

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Re: GROUP vs UNION

nophead
Are overlapping shells allowed in an STL? Surely that is one of the non-manifold things that upsets slicers.

On 7 February 2012 23:56, Gordon Wrigley <[hidden email]> wrote:
It's a bit odd, what would happen if you then nested that inside another CSG operation?


On Wed, Feb 8, 2012 at 10:54 AM, Marius Kintel <[hidden email]> wrote:
On Feb 8, 2012, at 00:45 AM, Stony Smith wrote:

> I saw some discussion earlier, but was there ever any work done to allow
> exporting separate, overlapping shells to a STL?
>
This is not (yet) supported, but would be interesting to look into.

 -Marius

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Re: GROUP vs UNION

kintel
Administrator
In reply to this post by Gordon Wrigley
On Feb 8, 2012, at 00:56 AM, Gordon Wrigley wrote:

> It's a bit odd, what would happen if you then nested that inside another CSG operation?
>
That's what "look into" would try to figure out ;)

 -Marius


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Re: GROUP vs UNION

Alan Cox
In reply to this post by nophead
On Tue, 7 Feb 2012 23:57:38 +0000
nop head <[hidden email]> wrote:

> Are overlapping shells allowed in an STL? Surely that is one of the
> non-manifold things that upsets slicers.

3D printers might not like it but I don't see why STL doesn't allow it

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Re: GROUP vs UNION

nophead
Well STL files are intended to represent physical objects to be made on a 3D printer. Physical objects are always manifold. What is the meaning / use of a mesh with overlapping shells?

On 8 February 2012 00:13, Alan Cox <[hidden email]> wrote:
On Tue, 7 Feb 2012 23:57:38 +0000
nop head <[hidden email]> wrote:

> Are overlapping shells allowed in an STL? Surely that is one of the
> non-manifold things that upsets slicers.

3D printers might not like it but I don't see why STL doesn't allow it
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Re: GROUP vs UNION

Triffid Hunter
In reply to this post by Alan Cox
On Wed, Feb 8, 2012 at 11:13 AM, Alan Cox <[hidden email]> wrote:
On Tue, 7 Feb 2012 23:57:38 +0000
nop head <[hidden email]> wrote:

> Are overlapping shells allowed in an STL? Surely that is one of the
> non-manifold things that upsets slicers.

3D printers might not like it but I don't see why STL doesn't allow it

STL will allow anything, it's simply a list of triangles in 3d space.

That's one of the troubles our slicers have, there's no requirement for manifoldness.

I'm also curious as to what possible use you could have for deliberately creating a self-intersecting non-manifold object?
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Re: GROUP vs UNION

donbright
what does manifold mean?

On Tue, Feb 7, 2012 at 6:27 PM, Triffid Hunter <[hidden email]> wrote:

> On Wed, Feb 8, 2012 at 11:13 AM, Alan Cox <[hidden email]> wrote:
>>
>> On Tue, 7 Feb 2012 23:57:38 +0000
>> nop head <[hidden email]> wrote:
>>
>> > Are overlapping shells allowed in an STL? Surely that is one of the
>> > non-manifold things that upsets slicers.
>>
>> 3D printers might not like it but I don't see why STL doesn't allow it
>
>
> STL will allow anything, it's simply a list of triangles in 3d space.
>
> That's one of the troubles our slicers have, there's no requirement for
> manifoldness.
>
> I'm also curious as to what possible use you could have for deliberately
> creating a self-intersecting non-manifold object?
>
> _______________________________________________
> OpenSCAD mailing list
> [hidden email]
> http://rocklinux.net/mailman/listinfo/openscad
>

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Re: GROUP vs UNION

Gordon Wrigley
Basically that the mesh represents a physical object, so no holes, no intersecting bits and weird stuff like an edge that's shared by 3 triangles.

On Wed, Feb 8, 2012 at 11:42 AM, Don Bright <[hidden email]> wrote:
what does manifold mean?

On Tue, Feb 7, 2012 at 6:27 PM, Triffid Hunter <[hidden email]> wrote:
> On Wed, Feb 8, 2012 at 11:13 AM, Alan Cox <[hidden email]> wrote:
>>
>> On Tue, 7 Feb 2012 23:57:38 +0000
>> nop head <[hidden email]> wrote:
>>
>> > Are overlapping shells allowed in an STL? Surely that is one of the
>> > non-manifold things that upsets slicers.
>>
>> 3D printers might not like it but I don't see why STL doesn't allow it
>
>
> STL will allow anything, it's simply a list of triangles in 3d space.
>
> That's one of the troubles our slicers have, there's no requirement for
> manifoldness.
>
> I'm also curious as to what possible use you could have for deliberately
> creating a self-intersecting non-manifold object?
>
> _______________________________________________
> OpenSCAD mailing list
> [hidden email]
> http://rocklinux.net/mailman/listinfo/openscad
>
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Re: GROUP vs UNION

Triffid Hunter
In reply to this post by donbright
On Wed, Feb 8, 2012 at 11:42 AM, Don Bright <[hidden email]> wrote:
what does manifold mean?

manifold is the difference between an M.C. Escher artwork and the surface of a real object.

For an STL to be manifold, it must satisfy certain criteria:

1) no holes in the mesh
 1a) every edge is adjacent to exactly two triangles
2) every vertex has one "in" side and one "out"side
 2a) object corners must not share a vertex
3) no triangles overlap or intersect, they must only touch 3 other triangles at their edges.
4) all the triangles' normals point towards the outside
 4a) for the two triangles adjacent to every edge, if they were rotated about the edge so they laid in the same plane, their normals would be parallel
5) it is absolutely certain whether any given point is inside or outside - the triangles form a cohesive and well defined boundary between inside and outside of a 3d solid.

It is quite difficult to coax openscad to produce manifold STLs with complex models
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Re: GROUP vs UNION

stonysmith
In reply to this post by stonysmith
> Well STL files are intended to represent physical objects to be made on a
> 3D printer. Physical objects are always manifold. What is the meaning /
use
> of a mesh with overlapping shells?

Well.. as it happens.. my two recent questions are directly related to each
other.

I want to extrude a set of shapes from a DXF, then export them as 3d objects
(STL) so I can import them into Truespace to edit them.  Since many, many of
the edges would be co-planar, I don't want them welded together in a CSG
operation.

Doing the extrusion of DXF objects in Truespace is a manual, one shape at a
time operation, and can take hours for some of the drawings I work with.

Note: YES, it is very common for STLs to have multiple shells in them.  I
think my personal record is around 1200. <GRIN>   For what it's worth..
that's one of the things that MeshMedic at Shapeways handles fairly well..
merging separate shells into something that can be printed.


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Re: GROUP vs UNION

nophead
But the STL file is just a bunch of triangles. Would Truespace be able to separate them into discrete solids, or would it have the same problems that slicers do?

I think it would be a bad idea to change Openscad to produce non-manifold STL files where it currently makes manifold ones. It would break existing designs and lose one of the big advantages CSG has over mesh modellers.

On 8 February 2012 04:19, Stony Smith <[hidden email]> wrote:
> Well STL files are intended to represent physical objects to be made on a
> 3D printer. Physical objects are always manifold. What is the meaning /
use
> of a mesh with overlapping shells?

Well.. as it happens.. my two recent questions are directly related to each
other.

I want to extrude a set of shapes from a DXF, then export them as 3d objects
(STL) so I can import them into Truespace to edit them.  Since many, many of
the edges would be co-planar, I don't want them welded together in a CSG
operation.

Doing the extrusion of DXF objects in Truespace is a manual, one shape at a
time operation, and can take hours for some of the drawings I work with.

Note: YES, it is very common for STLs to have multiple shells in them.  I
think my personal record is around 1200. <GRIN>   For what it's worth..
that's one of the things that MeshMedic at Shapeways handles fairly well..
merging separate shells into something that can be printed.

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Re: GROUP vs UNION

Kliment Yanev
Meshmedic is a rebranded version of cloud.netfabb so running the same files though that should make them printable on our machines too.

On 02/08/2012 10:15 AM, nop head wrote:
But the STL file is just a bunch of triangles. Would Truespace be able to separate them into discrete solids, or would it have the same problems that slicers do?

I think it would be a bad idea to change Openscad to produce non-manifold STL files where it currently makes manifold ones. It would break existing designs and lose one of the big advantages CSG has over mesh modellers.

On 8 February 2012 04:19, Stony Smith <[hidden email]> wrote:
> Well STL files are intended to represent physical objects to be made on a
> 3D printer. Physical objects are always manifold. What is the meaning /
use
> of a mesh with overlapping shells?

Well.. as it happens.. my two recent questions are directly related to each
other.

I want to extrude a set of shapes from a DXF, then export them as 3d objects
(STL) so I can import them into Truespace to edit them.  Since many, many of
the edges would be co-planar, I don't want them welded together in a CSG
operation.

Doing the extrusion of DXF objects in Truespace is a manual, one shape at a
time operation, and can take hours for some of the drawings I work with.

Note: YES, it is very common for STLs to have multiple shells in them.  I
think my personal record is around 1200. <GRIN>   For what it's worth..
that's one of the things that MeshMedic at Shapeways handles fairly well..
merging separate shells into something that can be printed.

_______________________________________________
OpenSCAD mailing list
[hidden email]
http://rocklinux.net/mailman/listinfo/openscad

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Re: GROUP vs UNION

Alan Cox
In reply to this post by nophead
On Wed, 8 Feb 2012 00:18:28 +0000
nop head <[hidden email]> wrote:

> Well STL files are intended to represent physical objects to be made on a
> 3D printer. Physical objects are always manifold. What is the meaning / use
> of a mesh with overlapping shells?

Firstly physical objects are not always manifold.. solids are (almost,
but we'll leave that to the quantum physicists). Secondly a lot of other
things where STL is useful for modelling purposes are decidedly non
manifold (consider modelling clearances for a set of moving parts, the
parts *can* overlap providing they don't exist in the same place at the
same moment, so the set of clearances are non manifold when you model
their full extent)

You can do a lot more with an STL model that print widgets.