CGAL users discussion list

[Benjamin McWhorter <>]

Hi Sebastian,

  Sorry if I wasn't clear on my problem.  I have attached my original input 
.obj files and the resulting converted .off files.

The problem that I am having is this, while I get an intersection and a 
resulting join that is valid between 2 of the 3, CGAL does not find a valid 
and simple join between the result of that join and the other nef_polyhedron 
(hopefully thats clear).  My process for performing the join is like this:

convert .obj to .off
create nef_polyhedron from .off using the off_to_nef3 constructor.
test if there is an intersection between the two nef_polyhedron
if there is test if the result of a join between the two would give a simple 
and valid result
if so, join them.
I then convert back to polyhedron and write the output to a file using 
print_polyhedron_wavefront

The problem is that running the program on the attached 3 inputs gives 2 
resulting nef_polyhedron.  If I run the program again, (using the output from 
the first run as input), then it will join the two polyhedron (after 
discarding the facets you mentioned when constructing the Nef_polyhedron).

So I basically have two questions -- why is it that these nef_polyhedron do 
not get joined in the first place?
Second -- why is CGAL's output (from print_polyhedron_wavefront) is giving me 
Polyhedron with degeneracies, when I do not get that problem from the 
attached input -- only CGAL produced input.

I hope that all makes sense and as always I very much appreciate your help.

Thanks,
Ben

Attachment: 3.obj
Description: Binary data

Attachment: 2.obj
Description: Binary data

Attachment: 1.obj
Description: Binary data

Attachment: output5.OFF
Description: Binary data

Attachment: output4.OFF
Description: Binary data

Attachment: output3.OFF
Description: Binary data

On Jun 15, 2011, at 3:36 AM, Sebastien Loriot (GeometryFactory) wrote:

> 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
> 
> 
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