CGAL_error_msg -> CGAL_NEF_TRACEN to allow convex decomposition on particular input

[aothms]

Summary of Changes

Apologies for the programming by accident here as I don't understand the majority of the code here. However, I propose here changing one particular occurrence of CGAL_error_msg to CGAL_NEF_TRACEN , which may be warranted giving the surrounding lines. CGAL_error_msg gives an assertion error and terminates program (at least on my configuration) however the code seems to be allowed to fall through since a null handle is returned on the next line which is actually handled at the call site.

Data

nef.txt

Steps to reproduce

int main(int, char**) {
	std::ifstream ifs("nef.txt");
	CGAL::Nef_polyhedron_3<CGAL::Epeck> nef;
	ifs >> nef;
	CGAL::convex_decomposition_3(nef);
}

• Without proposed fix throws assertion violation
• With proposed fix runs (seemlingly) normal

Output

(the apparent self intersections, I think are rendering artefacts from the z offset of edge rendering in blender).

Release Management

• Affected package(s): Convex Decomposition of Polyhedra / 3D Boolean Operations on Nef Polyhedra
• Issue(s) solved (if any): -
• Feature/Small Feature (if any): -
• Link to compiled documentation: -
• License and copyright ownership: -

[sloriot]

Looking a bit in the debugger it seems that you are most probably affected by the issue mentioned in the comment here,
which is somehow a good thing since we have a case to debug it.

    // The ray here has an special property since it is shooted from the lowest
    // vertex in a shell, so it would be expected that the ray goes along the
    // interior of a volume before it hits a 2-skeleton element.
    // Unfortunately, it seems to be possible that several shells are incident
    // to this lowest vertex, and in consequence, the ray could also go along
    // an edge or a facet belonging to a different shell.
    // This fact invalidates the precondition of the get_visible_facet method,
    // (the ray must pierce the local view of the hit object in a sface).
    // This situation has not been analyzed and has to be verified. Granados.