CGAL users discussion list

[Rostislav Khlebnikov <>]

Hi Sebastien,

yes, if I use the exact stuff it works fine. Again, it just seems 
strange that double is "not good enough" - it's not like one of 
triangles is 10 million times larger than the other or the intersection 
is almost nil (it is a segment with coordinates {6.791 3.52889 27.5591}, 
{6.791 6.82325 45.9274}).

Rostislav.

On 21/07/2014 06:46, Sebastien Loriot (GeometryFactory) wrote:
> What if you are using CGAL::Exact_predicates_exact_constructions_kernel?
>
> See also:
> http://www.cgal.org/FAQ.html#inexact_NT
>
> Sebastien.
>
> On 07/20/2014 08:22 PM, Rostislav Khlebnikov wrote:
> > Hello guys,
> >
> > I'm having the intersection procedure not returning correct results for
> > triangle-triangle intersection in 3D. I have compile a minimal example
> > that shows the problem:
> >
> > -----------------------------------------------------------------
> > #include <CGAL/Simple_cartesian.h>
> >
> > typedef CGAL::Simple_cartesian<double> K;
> > typedef K::FT FT;
> > typedef K::Point_3 Point;
> > typedef K::Triangle_3 Triangle;
> > typedef K::Segment_3 Segment;
> >
> >
> > void main()
> > {
> >      Triangle tri1 (Point(6.7910000000, -1.09375, -0.900025),
> > Point(6.791, 18.9, -0.9), Point(6.791, -1.09375, 144.321));
> >      Triangle tri1a(Point(6.7910000001, -1.09375, -0.900025),
> > Point(6.791, 18.9, -0.9), Point(6.791, -1.09375, 144.321));
> >
> >      Triangle tri2(Point(7.8, 5.6, 6.7269), Point(5.9, 1.7, 45.955),
> > Point(7.9, 13.2, 45.893));
> >
> >      std::cout << CGAL::intersection(tri1, tri2) << " " <<
> > CGAL::intersection(tri1a, tri2);
> > }
> > -----------------------------------------------------------------
> >
> > The output of this code is "0 1" which is quite strange. Do you think it
> > is a bug for some special case or is it the floating-point problem? The
> > thing is - I don't feel like the values are big or small enough for
> > "double" to be insufficient. Is there anything I can do to fix this
> > problem without changing to Exact_predicates_exact_constructions_kernel?
> > I would really like to avoid this - I need performance over accuracy -
> > but I feel that in this case the accuracy is far off what one would
> > expect. I could even detect this special case and add 1e-8 or whateve to
> > one of the points - it just feels weird and looks like a bug to me.
> >
> > CGAL installation info:
> > Windows 8.1, MSVS 2013, x64
> > CGAL 4.4 installed with the CGAL-4.4-Setup.exe installer and then built
> > with CMake and MSVS 2013.
> >
> > All best,
> >    Rostislav.