Render differs from Preview

Hello all,
I have a script that imports two stl's made in OpenSCAD and diffs them
from a cube with an end goal of exporting an stl to 3d print.  The
preview looks as expected, but when I render, one of the two stl's
disappears.  I started with 2019.05.  Now I've upgraded to 2020.12 rc3
and get the same behavior, but sometimes it gives an error message...

preview:
https://drive.google.com/file/d/1ajMFNMW8681kfhhqmL-dN1BFt7__YMFz/view?usp=sharing

render:
https://drive.google.com/file/d/1zKak3uN0M2360tiPQn1aAjpok33zZGsh/view?usp=sharing

error: ERROR: CGAL error in CGAL_Nef_polyhedron3(): CGAL ERROR:
assertion violation! Expr: e_below != SHalfedge_handle() File:
/usr/include/CGAL/Nef_3/SNC_FM_decorator.h Line: 426, location unknown

While playing with it, I added the cut-away and the object in the
foreground.  It's basically the same object that I'm having trouble
with.  It's made from the same scad model with the same shell script,
but with different dimensions passed through.  It doesn't cause any
errors, but the other one does.

Any ideas of how to fix this?

best regards,

Mike

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goblinbot wrote
> The preview looks as expected, but when I render, one of the two stl's
> disappears.
>
> error: ERROR: CGAL error in CGAL_Nef_polyhedron3(): CGAL ERROR:
> assertion violation! Expr: e_below != SHalfedge_handle() File:
> /usr/include/CGAL/Nef_3/SNC_FM_decorator.h Line: 426, location unknown

CGAL ERROR: assertion violation! - means 99.9% that a STL file is bad.

OpenSCAD can produce bad STLs.

If you share the STL files I can check them, then look at the relevant .scad
code.



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