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

Michael Marx michael at marx.id.au
Sat Nov 16 18:33:19 EST 2019


> So, if there was a mode/option/parameter that said 'touch'=='join', is there a down side to
solidify an edge/point to create a solid join?

 

Seems like a horrible bodge to me. Much better to overlap the cubes if you want then joined or
separate them slightly if you don't.

 

This is coming from the 'make it easier' perspective for casual users (and experienced ones too).

It is easy to fix with two cubes, what gets me and others as seen in the Forums, is when it is a
co-incident edge or point in the guts of a complex model.

 

Take this for example (thing to insert into Dyson handstick wall-mount to hold multiple
accessories)

 

 

As Doug said 'STL can't represent curved surfaces, it can only represent a polyhedral
approximation'.

That thing has so many curved surfaces [aka flat rectangles at angles] interfacing to each other
and actual flat surfaces, that in it's development I frequently ran into non-manifold conditions,
and also due to (I believe) the small number problem, producing self intersections.

 

You do some modelling, render, sometimes get the OpenSCAD non-manifold warning, or don't and
export, load it into Netfabb and get the dreaded triangle icon.

Sometimes even a small change in $fn will change alignment of edges and introduce the problem.

Yes there are techniques to reduce the likelihood, but they take a lot of experience to get to that
point.

 

I then have to drill into the area of the model, work out which cylinder's edge (usual culprit) has
managed to line-up to something. 

 

That thing is also made up of parts, which get positioned and joined to form various
configurations.

e.g. (thing to screw to wall to hold Dyson accessories with a large head, at an angle to it clears
the wall)



 

See all those curves meeting in the middle?



Or rather all those edges at angles?

It took me days to work out how to get a clean version of that.

 

If co-incident edges can be detected and (yes) 'fudged' with a little teeny-weeny sub-micron solid
glue it would make life much easier...

(In my experience it is usually only one or two edges causing the issue)

 

Isn't the whole idea of software to have it do the hard work for you?

 

Doug's wrong about some other export formats not having the problem BTW, so that can't fix it. But
that's another post...

 

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