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You need to use an exact-predicate and exact-construction kernel. If  
you do that and the problem still occurs, please send the entire  
(small) program that reproduces the problem.

Quoting 
:

> Hello,
>
> I eventually have found that the planes are being taken in the right way
> by dereferencing the iterator, but the program is failing in giving the
> right envelope, and giving an unexplainable exception, I started to
> believe that the algorithm has a problem... here's the part of the code,
> where P is a closed polyhedron object
>
>
>     for(plane_it = P.planes_begin(); plane_it != P.planes_end();++plane_it)
>     {
>         planes.push_back(Surface_3(*plane_it));
>         cout<<*plane_it<<endl;
>     }
>
>     /*
>     //this is a part where the example inserted the points
>     planes.push_back (Surface_3(Plane_3(-1, 0, 1, 0)));
>     planes.push_back (Surface_3(Plane_3(0, 1 , 1, 0)));
>     planes.push_back (Surface_3(Plane_3(1, 0, 1,  0)));
>     */
>
>       // Compute and print the minimization diagram.
>     Envelope_diagram_2      min_diag;
>
>     CGAL::lower_envelope_3 (planes.begin(), planes.end(), min_diag);
>
>     std::cout << std::endl << "The minimization diagram:" << std::endl;
>     print_diagram (min_diag);
>
>     // Compute and print the maximization diagram.
>     Envelope_diagram_2      max_diag;
>
>     CGAL::upper_envelope_3 (planes.begin(), planes.end(), max_diag);
>
>     std::cout << std::endl << "The maximization diagram:" << std::endl;
>     print_diagram (max_diag);
>
> The polyhedron I'm using has like 30 facets or more, and is closed and
> follow the Euler characteristic, and I could view it using Geomview. So
> I'm very confused and don't know where the error could be. The exception
> differs everytime because I'm every time generating a new polyhedron that
> fulfills the conditions mention. here's the error I'm getting in Terminal:
>
> terminate called after throwing an instance of 'CGAL::Precondition_exception'
>   what():  CGAL ERROR: precondition violation!
> Expr: cv.has_left()
> File: /usr/local/include/CGAL/Arr_linear_traits_2.h
> Line: 682
> Aborted
>
> and here's another one too:
> terminate called after throwing an instance of 'CGAL::Precondition_exception'
>   what():  CGAL ERROR: precondition violation!
> Expr: f == f2
> File: /usr/local/include/CGAL/Arrangement_2/Arrangement_on_surface_2_impl.h
> Line: 2639
> Explanation: The two halfedges must share the same incident face.
> Aborted
>
> This last error I can't understand, what does the envelope thing have to
> do with the halfedges? I'm giving the program plane equations and it has
> nothing to do with my polyhedron.
>
> Any effort is highly appreciated,
>
> Thank you,
> Samer Afach
>
>
>
>
> > Hello programmers,
> >
> > I have a question since I'm not a professional C++ programmer. I'm
> > trying to apply the 3D envelope example 32.3.3 in
> >
> > http://www.cgal.org/Manual/last/doc_html/cgal_manual/Envelope_3/Chapter_main.html
> >
> > but I'm taking the planes from a polyhedron, and I have them in the type
> > Plane_3 through the iterator Polyhedron::begin() to end(). The example
> > takes them in the form Surface_3(Plane_3()), how can I do this
> > transformation? I tried to use std::transform for this but I failed.
> > The example provides it using the 4 coefficients of a plane, I can see
> > these coefficients of my Plane_3 object on a cout stream, but I can't
> > extract them alone one by one.
> >
> > Could you please either tell me the correct transformation method for
> > this or tell me how to extract the coefficients from my Plane_3 object?
> >
> > Thank you,
> > Samer Afach
> >
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>
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