CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

See:

http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Kernel_23_ref/Function_intersection.html#Index_anchor_446

and in particular the part:
"With the 3D Spherical Kernel  (see Chapter 14) "

The object should be:
std::pair<Circular_arc_point_3<SphericalKernel>, unsigned>, where the 
unsigned integer is the multiplicity of the corresponding intersection 
point between obj1 and obj2,


yanyajie wrote:
> Dear S.
>  
> I fixed the problem by sustituting int with double in the 
> algebraic_kernel, but intersection routine just doesn’t return objects 
> whose type is what I expect. In the following code, the intersection 
> definitely happens, and two objects are inserted in vector isecs. If 
> nothing goes wrong, they should be two Point_3_SK instances, right? 
> However, object_cast<Point_3_SK> only yields null pointer.
> -------------------------------------------------------------------------------------
>     typedef SphericalK::Line_3 Line_3_SK;
>     typedef SphericalK::Sphere_3 Sphere_3_SK;
>     Line_3_SK line (Point_3_SK(0, 0, 0), Point_3_SK(0, 0, 1));
>     Sphere_3_SK sphere(Point_3_SK(0, 0, 0), 5);
>     vector<CGAL::Object> isecs;
>     CGAL::intersection(line, sphere, std::back_inserter(isecs));
>     const Point_3_SK *isec_p = CGAL::object_cast<Point_3_SK>(&isecs[0]);
> -------------------------------------------------------------------------------------
>  
> Where does the problem lie?
>  
> Thank you for your time!
>  
>  
> Yan.
>  
> *From:* yanyajie <mailto:>
> *Sent:* Friday, May 13, 2011 10:32 AM
> *To:*  
> <mailto:>
> *Subject:* Re: [cgal-discuss] precondition violation when intersecting a 
> line with a sphere
>  
> Dear S.
>  
> O! I was so naïve, as I thought “int” should be the type used to 
> instantiate Algebraic_kernel when I saw a RT in
>
>
>     CGAL::Algebraic_kernel_for_spheres_2_3<RT>
>
> OK, thank you! I’ll see if this correction could solve the problem.
>  
> Yan.
>  
> -----原始邮件-----
> From: Sebastien Loriot (GeometryFactory)
> Sent: Friday, May 13, 2011 1:45 AM
> To: 
>
> Subject: Re: [cgal-discuss] precondition violation when intersecting a 
> line with a sphere
>  
> Algebraic_kernel_for_spheres_2_3<int>
> should be Algebraic_kernel_for_spheres_2_3<double>
>  
> You also should consider using an exact type.
> Please read:
> http://www.cgal.org/FAQ.html#inexact_NT
>  
>  
> yanyajie wrote:
>  > Hi,
>  > 
>  > I encounterd a precondition violation error when I was trying to find
>  > the intersection between a line and a sphere.
>  > 
> -----------------------------------------------------------------------------------
>  > CGAL error: precondition violation!
>  > Expression : !p.degenerated()
>  > File       :
>  > d:\codes\lib\cgal-3.7\include\cgal\algebraic_kernel_for_spheres\int
>  > ernal_functions_on_roots_and_polynomial_1_3_and_2_3.h
>  > Line       : 212
>  > Explanation:
>  > Refer to the bug-reporting instructions at
>  > http://www.cgal.org/bug_report.html
>  > 
> -------------------------------------------------------------------------------------
>  > 
>  > I provide necessary info in the following to help reproduce the error:
>  > 
>  > 
> ------------------------------------------------------------------------------------------------------------------------------------------------------
>  > typedef CGAL::Cartesian<double> K;
>  > typedef CGAL::Spherical_kernel_3<K,
>  > CGAL::Algebraic_kernel_for_spheres_2_3<int> > SphericalK;
>  > typedef SphericalK::Point_3 Point_3_SK;
>  > 
>  > // to compute the intersection, I construct a line and a sphere using
>  > the spherical kernel as follows:
>  > CGAL::Line_3<SphericalK> line(Point_3_SK(-0.1,-0.1,-0.1),
>  > Point_3_SK(-0.1,-0.1,0.0));
>  > CGAL::Sphere_3<SphericalK> s(Point_3_SK(0, 0, 0), 0.1); // c is a Point_3
>  > vector<CGAL::Object> isecs;
>  > CGAL::intersection(line, s, std::back_inserter(isecs));
>  > 
> ------------------------------------------------------------------------------------------------------------------------------------------------------
>  > 
>  > The assertion error is issued from within CGAL::intersection, when it
>  > tries to do the confirmation,
>  > actually, from this function:
>  > 
>  > template < class AK, class OutputIterator >
>  > inline
>  > OutputIterator
>  >     solve(const typename AK::Polynomials_for_line_3 &p,
>  >           const typename AK::Polynomial_for_spheres_2_3& s,
>  >      OutputIterator res )
>  > {
>  > ...
>  > // we must have a line
>  > CGAL_kernel_precondition(!p.degenerated());
>  > ...
>  > }
>  > 
>  > 
>  > Does this failure of assertion indicates I constructed a degenerate
>  > line? But from the code you can see the line should be non-degenerate.
>  > 
>  > Thanks if anyone can point out the problem!
>  > 
>  > 
>  > Yan.
>  
>  
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