[OpenSCAD] Discuss manifoldness, co-incident faces edges etc

Michael Marx michael at marx.id.au
Sun Nov 17 08:40:22 EST 2019


> When you drive down a road and the road splits in two like a Y, does it
> matter whether one or the other arms of the Y is considered a
> continuation of the road you were driving on?

If it results in driving in-between the left & the right, yes it matters. Air-bags!

But it does go to intent. 
So you say it doesn't matter? 
What did the creator want?
One of the choices will be wrong. 
UNLESS the behaviour is deterministic.

> There is no problem with this
> case whether the edge is shared or duplicated, the slicer task is
> equally well defined in both cases.

So, which slicers make an outside corner, making them one object, or otherwise?

> Topologically, if there is a shared edge, it is one mesh.

So how thick is this joint? One (each side) perimeter of printer-nozzle dependent thickness?

Sorry, I retreated to - physical implementation stereotypes. Which printers make a solid, v's two
cubes? Or two-cubes with a crusty fragile edge?

Again, indeterminate. I'm suggesting jumping off the fence, to a predetermined side.

> > I'm suggesting that OpenSCAD remove the ambiguity of touching
> > non-manifolds too, by adding the smallest amount of solid to the edge.
> 
> It is not required and it is not solving anything.

Well it would save me lots of debugging & let me load it to commercial printing agencies and print
it without non-manifold objections.





> -----Original Message-----
> From: Discuss [mailto:discuss-bounces at lists.openscad.org] On Behalf Of Carsten Arnholm
> Sent: Sun, 17 Nov 2019 23:55
> To: discuss at lists.openscad.org
> Subject: Re: [OpenSCAD] Discuss manifoldness, co-incident faces edges etc
> 
> Discussing 2 cubes sharing an edge:
> 
> On 17.11.2019 04:19, Michael Marx wrote:
> > So the problem is still there. A slicer would still need to figure out
> > whether to do a left or right turn in the middle.
> 
> No it does not have such a dilemma.
> 
> I would say it is a misunderstanding to present this as a problem. There
> is nothing wrong with this model, it is just not 2-manifold everywhere.
> 1-manifold edges are a problem, but this 4-manifold edge is not.
> 
> The 2 subvolumes are well defined, and that is all that is required by a
> slicer. A slicer works by slicing (surprise!) the model into horizontal
> cuts with a constant thickness equal to the plastic extrusion height.
> The software needs to determine the polygons to be filled with plastic
> and do it using plastic extrusion paths. There is no problem with this
> case whether the edge is shared or duplicated, the slicer task is
> equally well defined in both cases.
> 
> > To me (not someone who studies this topic mathematically) it boils down
> > to: is it two objects or one?
> 
> That question is purely philosophical. It does not matter from a slicer
> point of view. Topologically, if there is a shared edge, it is one mesh.
> 
> > What says that the surface of one cube is a continuation of the surface
> > of the other? Or not?
> 
> That is of no interest to a slicer or the question of sharing models
> between applications.
> 
> When you drive down a road and the road splits in two like a Y, does it
> matter whether one or the other arms of the Y is considered a
> continuation of the road you were driving on?
> 
> > Isn't it inherently ambiguous?
> 
> The answer is no.
> 
> > So something has to be done to consider it one way or the other.
> 
> No, that is not necessary.
> 
> > I'm suggesting that OpenSCAD remove the ambiguity of touching
> > non-manifolds too, by adding the smallest amount of solid to the edge.
> 
> It is not required and it is not solving anything. Certainly it would
> create new problems if you tried.
> 
> Carsten Arnholm
> 
> _______________________________________________
> OpenSCAD mailing list
> Discuss at lists.openscad.org
> http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org




More information about the Discuss mailing list