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

Jordan Brown openscad at jordan.maileater.net
Mon Nov 18 13:42:13 EST 2019

On 11/16/2019 7:19 PM, Michael Marx wrote:
> To me (not someone who studies this topic mathematically) it boils
> down to: is it two objects or one?
> What says that the surface of one cube is a continuation of the
> surface of the other? Or not?

And for me that then boils down to "why do you care?".

If they overlap by a micrometer, the strongest substance in the world is
still going to have an awfully weak joint.  You better not be expecting
it to survive; if you want a joint you better make it thicker.

If they are separated by a micrometer, the best 3d printer in the world
is likely to still have them touch a bit, and so have an awfully weak
joint.  You better not expect them to be separate; if you want them to
be separate you better make the gap bigger.

All values of separation / overlap below a tenth of a millimeter or
so[*] are the same:  they might touch, or they might not.

    [*] If your reaction is "but my printer can do that level of
    accuracy", divide by ten and restart.

If they overlap by a micrometer, is the slicer going to say "that's too
small to print", and print as two perimeters, or is it going to say
"every part of the model deserves to be printed, even if we have to
overflow the bounds to print it", and print as one perimeter?  Once
you've answered that question, great, apply that answer to the "zero"
case and I'll be happy.

(Abstractly, I like the "color inside the lines" answer.  Never let the
plastic out of the lines; if the space between the lines is less than
the minimum extrusion width, don't print.  Practically, I like the
"overflow" answer, so that if I scale a model down small things turn
into hairlines rather than disappearing.  I have a small model
<https://xkcd.com/878/> where this is important, where I have rectangles
that are about 0.25mm thick.  Either answer is reasonable.  Probably it
should be a configurable item on the slicer.)

I understand that there may be algorithmic problems with the "zero"
case.  But I can't see how there are any practical problems.

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