[OpenSCAD] CGAL error depends on order of unioning

Tim V. Shaporev tim.shaporev at auriga.ru
Wed Jul 24 08:49:15 EDT 2019

Let me put it different way: is it documented anywhere
1) that all intermediate results should be manifold and
2) exact order of operations (note that enclosing quarters in additional 
explicit - or implicit - union like in 'good1' resolves the issue).
If these are documented then it is my fault that I did not read it 
(though I'd like to have the issue fixed as an enhancement).

But as far as the final geometry is correct I see this as a bug
(though I do understand that it could be too hard to fix).

And, finally, thank you for the explanation - it is helpful.


On 7/23/2019 12:36 PM, nop head wrote:
> It probably isn't a bug. It is quite possible to have three objects such 
> that unioning two of them gives a non-manifold result but unioning them 
> in another order doesn't. For example consider two cubes sharing one 
> edge and a third that overlaps that edge. If you do the union one, two, 
> three you will get an error but one, three, two will work and so will 
> any other order because the intermediate results are all manifold.
> On Tue, 23 Jul 2019 at 10:08, Shaporev, Timur <tim at auriga.com 
> <mailto:tim at auriga.com>> wrote:
>     While working on https://www.thingiverse.com/thing:3762730 I faced
>     strange effect: script produce
>     ERROR: CGAL error in CGALUtils::applyBinaryOperator union: CGAL
>     ERROR: assertion violation! Expr: itl != it->second.end() File:
>     /mxe/usr/x86_64-w64-mingw32.static.posix/include/CGAL/Nef_3/SNC_external_structure.h
>     Line: 1152
>     on F6 but some change in order of unioning yields expected result.
>     I am attaching an archive with both faulty and good versions of scripts.
>     (The scripts render long enough: more than 20 minutes to preview the
>     first script on my PC and 6 more minutes to render, yet more for the
>     second script.)
>     If this is new issue I'd appreciate advice on how to (get a ticket
>     number) register it.
>     Thank you,
>     Tim
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