CGAL users discussion list

[Stefan Schirra <>]

Hi,

please have a look at the documentation of CGAL::to_double().
It returns a double approximation of a number, but not necessarily a 
best possible. There are no general guarantees whatsoever.

Best regards

        Stefan


Am 03/11/2023 um 15:20 schrieb Alessio Mochi ( via 
cgal-discuss Mailing List):
> Hello, I trying to do a circle-circle intersection algorithm. The 
> problem is that the function gives a correct result only when the 
> coordinates of the circle centers and the circle radii are integers.
>
> |#include <CGAL/Exact_predicates_exact_constructions_kernel.h> #include 
> <CGAL/Exact_circular_kernel_2.h> #include <CGAL/intersections.h> 
> #include <iostream> typedef CGAL::Exact_circular_kernel_2 Circular_k; 
> typedef CGAL::Point_2<Circular_k> Point_2k; typedef 
> CGAL::Circle_2<Circular_k> Circle_2k; typedef 
> CGAL::Circular_arc_2<Circular_k> Circular_arc_2; typedef 
> CGAL::CK2_Intersection_traits<Circular_k, Circle_2k, Circle_2k>::type 
> Intersection_result; struct P2 { double x; double y; }; struct 
> CGALCircleCircleInterface { bool result; P2 pout[2]; }; int main() { 
> CGALCircleCircleInterface circle_circle_Interface; //Point2d(10.5, 0), 
> 0.3, Point2d(10.8, 0), 0.4 double circle1_x = 10.5; double circle1_y = 
> 0; double circle2_x = 10.8; double circle2_y = 0; double ray1 = 0.3; 
> double ray2= 0.4; // first circle with center (circle1_x, circle1_y) and 
> radius ray1 Circle_2k c1(Point_2k(circle1_x, circle1_y), ray1); 
> Circle_2k c2(Point_2k(circle2_x, circle2_y), ray2); Point_2k p; // Store 
> intersection points std::vector<Intersection_result> res; // calculate 
> intersection CGAL::intersection(c1, c2, std::back_inserter(res)); using 
> boostRetVal = 
> std::pair<CGAL::Circular_arc_point_2<CGAL::Filtered_bbox_circular_kernel_2<CGAL::Circular_kernel_2<CGAL::Cartesian<CGAL::Gmpq>, 
> CGAL::Algebraic_kernel_for_circles_2_2<CGAL::Gmpq> > > >, unsigned>; if 
> (res.empty()) circle_circle_Interface.result = false; else 
> circle_circle_Interface.result = true; for (int i = 0; i < res.size(); 
> i++) { auto algPoint = std::get<0>(boost::get< boostRetVal >(res[i])); 
> auto point = Point_2k(to_double(algPoint.x()), to_double(algPoint.y())); 
> circle_circle_Interface.pout[i].x = CGAL::to_double(point.x()); 
> circle_circle_Interface.pout[i].y = CGAL::to_double(point.y()); } } |
>
> Th result of two intersections points store in structure 
> CGALCircleCircleInterface get bad values.
>
> |bad values algorithm: {10.483333333333333, -0.54746892352189469} 
> {10.483333333333333, 0.54746892352189469} correct values: 
> {10.533333333333333, 0.29814239699997191} {10.533333333333333, 
> -0.29814239699997191} |
>
>
>
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