CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

IIRC, the intersection of the supporting planes if first computed and 
the line is then intersected with one of the triangles.
In this setting, I'm not surprised you didn't get the correct result.
You could use some filtered construction when the intersection would be
computed with EPECK and then converter back to the kernel with inexact
construction.

Sebastien.

On 07/21/2014 01:31 PM, Rostislav Khlebnikov wrote:
> 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.