CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

Read these FAQ entries:
http://www.cgal.org/FAQ.html#predicates_vs_constructions
http://www.cgal.org/FAQ.html#inexact_NT

If you replace Exact_predicates_inexact_constructions_kernel by
Exact_predicates_exact_constructions_kernel your error won't appear.

Sebastien.


On 06/18/2012 11:52 PM, W Randolph Franklin wrote:
> Hi,
>
> The following program fragment seems to show that the intersection of 2
> segments can be outside their bounds.   I'm still learning, so the
> mistake is probably mine, but I can't find it.  Help?
>
> FYI, error.C was extracted from a much larger program and dataset.
>
> Note that I'm using Exact_predicates_inexact_constructions_kernel and
> the erroneous result was computed with a chain of Segment_2,
> intersection, compare_x.
>
> I compiled it this:
>
> /usr/bin/g++-4.7 -O3 -frounding-math     error.C  -lCGAL -lmpfr -lgmp -o
> error
>
> My system is this:
>
> Linux host 3.2.0-25-generic #40-Ubuntu SMP Wed May 23 20:30:51 UTC 2012
> x86_64 x86_64 x86_64 GNU/Linux
>
> Thanks.
>
> ================================================================
>
> // Time-stamp:</wrf/c/overprop/error.C, Mon, 18 Jun 2012, 17:41:12 EDT,
> http://wrfranklin.org/>
>
> #include<iomanip>
> #include<iostream>
> using namespace std;
>
> #include<CGAL/Exact_predicates_inexact_constructions_kernel.h>
> #include<CGAL/intersections.h>
> #include<CGAL/Object.h>
> typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
> using CGAL::Object;
> using CGAL::compare_x;
> using CGAL::compare_y;
> using CGAL::object_cast;
>
> // Print an expression's name then its value, possible followed by a
> // comma or endl.  Ex: cout<<  PRINTC(x)<<  PRINTN(y);
> #define PRINT(arg)  #arg "="<<  (arg)
> #define PRINTC(arg)  #arg "="<<  (arg)<<  ","
> #define PRINTN(arg)  #arg "="<<  (arg)<<  endl
>
> int main() {
>    cout<<  "Intersection of 2 segments is outside their bounds."<<  endl;
>    K::Point_2 v0(63.6421, 2067.410);
>    K::Point_2 v1(62.7046, 2071.750);
>    K::Point_2 w0(62.7046, 2071.750);
>    K::Point_2 w1(59.8669, 2066.980);
>    Object obj = CGAL::intersection(K::Segment_2(v0,v1), K::Segment_2(w0,w1));
>    if (const K::Point_2 *xsect = object_cast<K::Point_2>(&obj)) {
>      cout<<  "Intersection found. "<<  PRINTC(v0)<<  PRINTN(v1)<<
> PRINTC(w0)<<  PRINTN(w1)<<  PRINTN(*xsect);
>      if ((compare_x(*xsect,v0)*compare_x(*xsect,v1)>0) ||
>         (compare_y(*xsect,v0)*compare_y(*xsect,v1)>0) ||
>         (compare_x(*xsect,w0)*compare_x(*xsect,w1)>0) ||
>         (compare_y(*xsect,w0)*compare_y(*xsect,w1)>0)) {
>        cout<<  "\nERROR: intersection is not within bounds of edges."<<
> endl;
>        cout<<  PRINTN(compare_x(*xsect,v0));
>        cout<<  PRINTN(compare_x(*xsect,v1));
>        cout<<  PRINTN(compare_y(*xsect,v0));
>        cout<<  PRINTN(compare_y(*xsect,v1));
>        cout<<  PRINTC(compare_x(*xsect,w0))<<  " WRONG, should be 0"<<  endl;
>        cout<<  PRINTN(compare_x(*xsect,w1));
>        cout<<  PRINTN(compare_y(*xsect,w0));
>        cout<<  PRINTN(compare_y(*xsect,w1));
>        cout<<  scientific<<  PRINTN(xsect->x()-w0.x())<<  endl;
>      }
>    }
> }
>
> ================================================================
> Output:
>
> Intersection of 2 segments is outside their bounds.
> Intersection found. v0=63.6421 2067.41, v1=62.7046 2071.75
> w0=62.7046 2071.75, w1=59.8669 2066.98
> *xsect=62.7046 2071.75
>
> ERROR: intersection is not within bounds of edges.
> compare_x(*xsect,v0)=-1
> compare_x(*xsect,v1)=1
> compare_y(*xsect,v0)=1
> compare_y(*xsect,v1)=0
> compare_x(*xsect,w0)=1,  WRONG, should be 0
> compare_x(*xsect,w1)=1
> compare_y(*xsect,w0)=0
> compare_y(*xsect,w1)=1
> xsect->x()-w0.x()=7.105427e-15
>
> ================================================================
>
>
> --
> W. Randolph Franklin, Professor
> ECSE Dept., 6026 JEC,
> Rensselaer Polytechnic Inst,
> 110 8th St,
> Troy NY, 12180 USA
>
>
>  (most reliable address) or 
>
>  or
>
>
> +1 (518) 276-6077
>
> http://wrfranklin.org/