CGAL users discussion list

[Benjamin Kehlet <>]

Hello!

I am attempting to use the 2D snap rounding package to safely get from
exact numbers to float in our mesh generation tool in FEniCS. However,
I get some (to me) unexpected results. Here is a simple example:
When running this simple modification of
examples/Snap_rounding_2/snap_rounding.cpp:
---
#include <CGAL/Cartesian.h>
#include <CGAL/Quotient.h>
#include <CGAL/MP_Float.h>
#include <CGAL/Snap_rounding_traits_2.h>
#include <CGAL/Snap_rounding_2.h>

typedef CGAL::Quotient<CGAL::MP_Float>           Number_type;
typedef CGAL::Cartesian<Number_type>             Kernel;
typedef CGAL::Snap_rounding_traits_2<Kernel>     Traits;
typedef Kernel::Segment_2                        Segment_2;
typedef Kernel::Point_2                          Point_2;
typedef std::list<Segment_2>                     Segment_list_2;
typedef std::list<Point_2>                       Polyline_2;
typedef std::list<Polyline_2>                    Polyline_list_2;

int main()
{
  Segment_list_2 seg_list;
  Polyline_list_2 output_list;

  seg_list.push_back(Segment_2(Point_2(1, 0), Point_2(0, 0)));

  // Generate an iterated snap-rounding representation, where the centers of
  // the hot pixels bear their original coordinates, using 5 kd trees:
  CGAL::snap_rounding_2<Traits,Segment_list_2::const_iterator,Polyline_list_2>
    (seg_list.begin(), seg_list.end(), output_list, .1, true, false, 5);

  int counter = 0;
  Polyline_list_2::const_iterator iter1;
  for (iter1 = output_list.begin(); iter1 != output_list.end(); ++iter1) {
    std::cout << "Polyline number " << ++counter << ":\n";
    Polyline_2::const_iterator iter2;
    for (iter2 = iter1->begin(); iter2 != iter1->end(); ++iter2)
      std::cout << "    (" << iter2->x() << ":" << iter2->y() << ")\n";
  }

  return(0);
}
---

I get this result:
benjamik@benjamik-ThinkPad-X300:~/software/CGAL-4.4/examples/Snap_rounding_2/build$
./snap_rounding
Polyline number 1:
    (0.1/2:0.1/2)
benjamik@benjamik-ThinkPad-X300:~/software/CGAL-4.4/examples/Snap_rounding_2/build$

So the one segment in the input seems to be "reduced" to a single
point. Is this expected? If the pixel size argument in the call to
CGAL::snap_rounding_2() is changed to "Number_type(1, 10)", then I get
the output that I would expect:

benjamik@benjamik-ThinkPad-X300:~/software/CGAL-4.4/examples/Snap_rounding_2/build$
./snap_rounding
Polyline number 1:
    (210/200:10/200)
    (10/200:10/200)
benjamik@benjamik-ThinkPad-X300:~/software/CGAL-4.4/examples/Snap_rounding_2/build$

This may be due to my limited knowledge of the
Quotient<CGAL::MP_float> number type, but from the reference manual it
seems that it should be constructable from a double.

This occurs with a debug build of CGAL 4.4. I get the equal results on
a 32-bit Ubuntu 13.10 box with gcc 4.8.1 and with a 64-bit debian
Jessie box with gcc 4.9.0.

Best regards

Benjamin Kehlet