CGAL users discussion list

[gupta61 <>]

Hi,

I am reporting a problem with the "Segment-Triangle" intersection. If you
look at the code below I am trying to find a intersection between a Segment
and a Triangle. The answer should be that they do not intersect. When I use
"do_intersect" to find if they intersect, it gives me the correct result.
However when I ask "intersection" to tell me if it has an intersection, then
it gives me a segment as the intersection which is obviously wrong. Let me
know if I am missing something. I am mentioning the various things I have
tried. 



I have tried with this problem with three versions of CGAL (3.8, 3.9,
4.0-beta1). 

 I also tried these kernels: 

#include <CGAL/Exact_predicates_exact_constructions_kernel.h>
typedef CGAL::Exact_predicates_exact_constructions_kernel       Kernel; 

#include <CGAL/Simple_cartesian.h>
typedef CGAL::Simple_cartesian<double>        Kernel; 

and got the same wrong results... 



I am also mentioning the code which we used for this. 


#include <cstdlib>
#include <string>
#include <cstdio>
      
#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
typedef CGAL::Exact_predicates_inexact_constructions_kernel     Kernel;

//#include <CGAL/Exact_predicates_exact_constructions_kernel.h>
//typedef CGAL::Exact_predicates_exact_constructions_kernel       Kernel;

//#include <CGAL/Simple_cartesian.h>
//typedef CGAL::Simple_cartesian<double>        Kernel;


#include <CGAL/intersections.h>
#include <CGAL/Bbox_3.h>

typedef CGAL::Point_3<Kernel>                 CGALPoint3;
typedef CGAL::Segment_3<Kernel>               CGALSegment3;
typedef CGAL::Triangle_3<Kernel>              CGALTriangle3;

int main(int argc, char** argv) {

  CGALSegment3 segment_query( CGALPoint3(0.125, 0, -0.125), 
                              CGALPoint3(0.25, 0, -0.125) );

  CGALTriangle3 triangle( CGALPoint3(0.2500001, 0, -0.125) /* vertex 6*/,
                          CGALPoint3(1.0278171, 0, -0.125) /* vertex 10*/,
                          CGALPoint3(1.0278171, 0, -0.250001) /* vertex 9*/ );

  std::cout << std::setw(12) << std::setprecision( 15 )
            << "\nSegment query: " << segment_query << std::endl;
  std::cout << std::setw(12) << std::setprecision( 15 )
            << "\nTriangle: " << triangle << std::endl;

  bool do_intersect =  CGAL::do_intersect(segment_query, triangle);
  std::cout << "\nCGAL::do_intersect (0/1) = " << do_intersect << std::endl;


  // Compute actual intersection(s)
  CGAL::Object object = CGAL::intersection(segment_query, triangle);

  // check if it is a point
  const CGALPoint3* p_point=0;
  const CGALSegment3* p_segm=0;
  if( (p_point = CGAL::object_cast<CGALPoint3>(&object)) ) {
    std::cout << "\nEncountered intersection object is a point" <<
std::endl;
    std::cout << std::setw(12) << std::setprecision( 15 ) << (*p_point) <<
std::endl;
  }
  else if ( (p_segm = CGAL::object_cast<CGALSegment3>(&object)) ) {
    // Check if it is a segment otherwise error out
         
    std::cout << "\nEncountered intersection object is a segment" <<
std::endl;
    std::cout << std::setw(12) << std::setprecision( 15 ) << (*p_segm) <<
std::endl;
  }
  else {
    std::cerr << "\nInvalid object returned by aaa_bb tree" << std::endl;
  }

   return 0;
}


We got the following result which is obviously wrong as explained before. 

Segment query: 0.125 0 -0.125 0.25 0 -0.125
 
Triangle: 0.2500001 0 -0.125 1.0278171 0 -0.125 1.0278171 0 -0.250001

CGAL::do_intersect (0/1) = 0

Encountered intersection object is a segment
       0.125 0 -0.125 0.25 0 -0.125


Can you explain why I am getting this result or if you can confirm if it is
a bug ? 

Thanks, 
Piyush Gupta 
UIUC

-----
Piyush Gupta
UIUC
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