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["Sebastien Loriot (GeometryFactory)" <>]

If you are using the construction from polyhedron,
nef assumes that the input is a valid polyhedron.
Your input output4.OFF contains facets made only of collinear
points.

The following code detect these facets:

for (Polyhedron::Facet_iterator f=poly.facets_begin();
     f!=poly.facets_end();++f)
  {
    Kernel::Point_3 pt1,pt2,pt3;
    pt1=f->halfedge()->vertex()->point();
    pt2=f->halfedge()->next()->vertex()->point();
    Polyhedron::Halfedge_handle h=f->halfedge()->next();
    do{
      pt3=f->halfedge()->next()->next()->vertex()->point();
      if (! CGAL::collinear(pt1,pt2,pt3) ) break;
      h=h->next();
    }
    while(h!=f->halfedge());
    if( h==f->halfedge() )
      std::cerr<<"Error degenerate facet" << std::endl;
  }

In order to remove this kind of degeneracies, you can use the
function CGAL::OFF_to_nef_3 documented here:
http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Nef_3_ref/Function_OFF_to_nef_3.html

Using this function I was able to make the union of your
inputs with CGAL-3.8.

Sebastien.


Benjamin McWhorter wrote:
> Hello All,
>
> I have been having some trouble correctly joining Nef_Polyhedron.  I have 
> attached some .off files that should recreate the error.  I have been trying 
> to join a large number of Polyhedron using CGAL and ran into many problems 
> along the way.  I found that I cannot simply join the simple and valid 
> nef_polyhedron and expect a simple and valid result.  The only way I was able 
> to accurately join them was by testing if the result of the join was simple 
> and valid after checking if there was an intersection and if it passed all 
> three tests, then perform the join.
>
> It seems that the order in which I joined them had a lot to do with it 
> working out.
>
> Anyhow, on to my actual issue.  I ran the join and expected the following two 
> polyhedron to be joined, but they were not.  After the join I convert back to 
> Polyhedron and then use the print_polyhedron_wavefront function to write the 
> output in an .obj file.
>
> These .OFF files that I attached are simply print_polyhedron_wavefront .objs 
> that I have converted to .OFF.  These two were not joined during execution, 
> however, when I ran the program again with these two as input, I get the 
> following (sorry if I put too much).  I am wondering why CGAL is discarding 
> facets that were created by the print_polyhedron_wavefront function and 2, 
> why these two were not joined in the first place.  Any help is much 
> appreciated.
>
> Thanks,
> Ben
>
>
>
> ------------------------------------------------------------------------
>
>
>
> output4.OFF
>
> Conversion from vertex cycle to Nef_polyhedron_3 was not successful. Error history: -> 
> function parameter 'normal' is NULL_VECTOR (this can be a symptom of an error). Now, 
> direction of projection is 'x'. -> Same vertex appears multiple in cycle. Now, 
> direction of projection is 'y'. -> Same vertex appears multiple in cycle. Now, 
> direction of projection is 'z'. -> Same vertex appears multiple in cycle. Finally, 
> empty Nef_polyhedron_3 was constructed.
> Hence, discard input facet 14 (enumerated beginning with 1). Check semantics!
>
>
> Conversion from vertex cycle to Nef_polyhedron_3 was not successful. Error history: -> 
> function parameter 'normal' is NULL_VECTOR (this can be a symptom of an error). Now, 
> direction of projection is 'x'. -> Same vertex appears multiple in cycle. Now, 
> direction of projection is 'y'. -> Same vertex appears multiple in cycle. Now, 
> direction of projection is 'z'. -> Same vertex appears multiple in cycle. Finally, 
> empty Nef_polyhedron_3 was constructed.
> Hence, discard input facet 21 (enumerated beginning with 1). Check semantics!
>
>
> Conversion from vertex cycle to Nef_polyhedron_3 was not successful. Error history: 
> -> function parameter 'normal' is NULL_VECTOR (this can be a symptom of an 
> error). Now, direction of projection is 'x'. Now, direction of projection is 'y'. 
> Now, direction of projection is 'z'. -> Different vertices are projected to same 
> location! Finally, empty Nef_polyhedron_3 was constructed.
> Hence, discard input facet 29 (enumerated beginning with 1). Check semantics!
>
>
> Conversion from vertex cycle to Nef_polyhedron_3 was not successful. Error history: -> 
> function parameter 'normal' is NULL_VECTOR (this can be a symptom of an error). Now, 
> direction of projection is 'x'. -> Same vertex appears multiple in cycle. Now, 
> direction of projection is 'y'. -> Same vertex appears multiple in cycle. Now, 
> direction of projection is 'z'. -> Different vertices are projected to same location! 
> Finally, empty Nef_polyhedron_3 was constructed.
> Hence, discard input facet 30 (enumerated beginning with 1). Check semantics!
>
>
> Conversion from vertex cycle to Nef_polyhedron_3 was not successful. Error history: -> 
> function parameter 'normal' is NULL_VECTOR (this can be a symptom of an error). Now, 
> direction of projection is 'x'. -> Same vertex appears multiple in cycle. Now, 
> direction of projection is 'y'. -> Same vertex appears multiple in cycle. Now, 
> direction of projection is 'z'. -> Different vertices are projected to same location! 
> Finally, empty Nef_polyhedron_3 was constructed.
> Hence, discard input facet 51 (enumerated beginning with 1). Check semantics!
>
> output4.OFF is simple
> output4.OFF is valid
> output3.OFF
> output3.OFF is simple
> output3.OFF is valid