CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

I tried the following:

include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <iostream>
#include <vector>

typedef CGAL::Exact_predicates_inexact_constructions_kernel K;

int main()
{
  K::Point_3 p1(-119.452959304302,54.727712609640498,888);
  K::Point_3 p2(-119.27352872835201,54.731184848162897,820);
  K::Point_3 p3(-119.369047730614,55.029360094556502,678);

  K::Vector_3 v = CGAL::unit_normal(p1, p2, p3);

  std::cout << v * v << std::endl;
}


and 1 is displayed.

Sebastien.

On 09/12/2016 12:52 AM, chrism wrote:
> I have a set of points in UTM and lat/long coordinates that represent the
> vertices of a triangle mesh located on the surface of the earth. I need to
> calculate a unit normal to this triangle, and so use the CGAL::unit_normal
> function.
>
> With UTM coordinates this works as expected.
>
> With lat/long/elevation the wrong answer is produced:
>
> (-119.452959304302,54.727712609640498,888)
> (-119.27352872835201,54.731184848162897,820)
> (-119.369047730614,55.029360094556502,678)
>
> If I scale the x,y by 100 000, it then works correctly:
>
> (-11945295.9304302,5472771.26096405,888)
> (-11927352.8728352,5473118.4848162895,820)
> (-11936904.7730614,5502936.0094556501,678)
>
> Strangely: without this scaling, paraview is unable to render the mesh
> properly (same issue?).
>
> If I plot these point sets in Wolfram Alpha, the problem is apparent: the
> non-scaled produces a plane that is vertical instead of horizontal.
>
> works
> https://www.wolframalpha.com/input/?i=plane+through+(-11945295.9304302,5472771.26096405,888)+(-11927352.8728352,5473118.4848162895,820)+(-11936904.7730614,5502936.0094556501,678)
> broken
> https://www.wolframalpha.com/input/?i=plane+through+(-119.452959304302,54.727712609640498,888)+(-119.27352872835201,54.731184848162897,820)+(-119.369047730614,55.029360094556502,678)
>
> I am using an Exact_predicates_inexact_constructions_kernel.
>
> I have tried with the exact constructions, but it doesn't seem to allow me
> to use it alongside the inexact?When I call the unit_normal function, I get
>
> /usr/include/CGAL/Kernel/function_objects.h:1879:17: error: no match for
> ‘operator/’ (operand types are
> ‘CGAL::CommonKernelFunctors::Construct_unit_normal_3<CGAL::Simple_cartesian&lt;CGAL::Gmpq>
> > ::Vector_3 {aka CGAL::Vector_3<CGAL::Simple_cartesian&lt;CGAL::Gmpq> >}’
> and ‘CGAL::Null_functor::result_type {aka CGAL::Null_tag}’)
>         res = res / CGAL::sqrt(res.squared_length());
>
> Any ideas what is causing the unit normal to be so incorrect? I'm guessing a
> numeric issue, but I'm not really sure where? Is there a way around this?
> It's a one off computation so can take the hit of exact constructions if
> that a) solves it and b) is used along side the inexact.
>
>
>
> --
> View this message in context: 
> http://cgal-discuss.949826.n4.nabble.com/Wrong-unit-normal-with-CGAL-unit-normal-tp4662261.html
> Sent from the cgal-discuss mailing list archive at Nabble.com.