Rounding errors could push us slightly outside the [-1, 1] range, which
caused acos() to return NaN, which in turn broke p2d_is_cw, and eventually
tripped create_interior_skeleton_and_offset_polygons_with_holes_2.
This broke p2d_is_cw which in turn tripped
CGAL::create_interior_skeleton_and_offset_polygons_with_holes_2
with the infamous
terminate called after throwing an instance of 'CGAL::Assertion_exception'
what(): CGAL ERROR: assertion violation!
Expr: lParent
File: /usr/include/CGAL/arrange_offset_polygons_2.h
Line: 94
cameo encountered spurious aborts with the following message:
terminate called after throwing an instance of 'CGAL::Precondition_exception
what(): CGAL ERROR: precondition violation!
Expr: is_simple_2(first, last, traits)
File: /usr/include/CGAL/Polygon_2/Polygon_2_algorithms_impl.h
Line: 420
These aborts come and go even if just the coordinate origin is slightly
changed. Setting CGAL_POLYGON_NO_PRECONDITIONS in p2d_area_holes.cpp
made them go away in some cases, but cameo could still run into an
assertion violation (in CGAL).
We now check all the polygons, outer boundary and holes, being sent to
CGAL::create_interior_skeleton_and_offset_polygons_with_holes_2 for
simplicity, and stop recursing if there is any hint of a problem.
This seems to avoid trouble for now, but the issue deserves further
investigation.
Werner Almesberger