CGAL users discussion list

[Monique Teillaud <>]

----- Le 25 Juin 19, à 11:55, Stefan Witzel 

 a écrit :

> Hi again,
> 
> I am working with a Hyperbolic Delaunay triangulation, more specifically 
> with a
> 
> CGAL::Hyperbolic_Delaunay_triangulation_2<CGAL::Hyperbolic_Delaunay_triangulation_traits_2<>>
> dt
> 
> Now I'm experiencing the following behavior: I have a program (and a
> set of input) that finishes "immediately" but if I add some code, it
> takes on the order of minutes and more *even in the old part of the
> code*. So clearly there is some side effect, and I'm pretty certain
> that I'm not producing it (the only thing I'm passing around is
> const). For all I can tell the output (of the old part of the code) is
> exactly the same as before so the side effect is restricted to
> runtime.
> So I'm wondering: is it possible that previous calculations performed
> with dt affect future calculations? I'm imagining something like if
> some calculation needed a certain precision (or number field), the
> consecutive calculations are performed with the same precision even
> though they might not need it. 

Hi,

Might be, but honestly without a crystal ball that would tell me what yo are 
doing, I cannot say more... 

> If this is the case, is there a way to "reset" the precision?

I don't know much about Core number types, probably the Core manual says 
something, see https://cs.nyu.edu/exact/core_pages/intro.html (however be 
aware that the version that we maintain in CGAL is not the same as the native 
Core)

> P.S.: In case someone has read the previous posts and is wondering: at
> the moment I can't get the circular kernel to work because I don't
> know how to turn a Hyperbolic_Voronoi_point_2 into a
> Hyperbolic_point_2.

I would recommend to have a quick look at the paper that we cite in the 
Design and Implementation History section: Mikhail Bogdanov, Olivier 
Devillers, and Monique Teillaud. Hyperbolic Delaunay complexes and Voronoi 
diagrams made practical. Journal of Computational Geometry, 5:56–85, 2014. 
http://hal.inria.fr/hal-00961390.

More details:
Hyperbolic_Voronoi_point_2 is the intersection of two circular arcs whose 
centers are two Hyperbolic_point_2. So, the coordinates of a 
Hyperbolic_Voronoi_point_2 lie in an extension field of degree 2, compared to 
the coordinates of a Hyperbolic_point_2. That's precisely why we have two 
types in CGAL. 
So, if you are using rational input points with 
CGAL::Hyperbolic_Delaunay_triangulation_CK_traits_2, you cannot convert 
Hyperbolic_Voronoi_point_2 into Hyperbolic_point_2. 
If you are using input points with algebraic coordinates and 
CGAL::Hyperbolic_Delaunay_triangulation_traits_2, I guess that the two point 
types are the same (to be checked in the code, they are probably both 
Point_2<CORE::Expr>). 

As mentioned in the manual, if you are just computing hyperbolic Delaunay 
triangulations and Voronoi diagrams of point sites having rational 
coordinates, then CGAL::Hyperbolic_Delaunay_triangulation_CK_traits_2 is 
enough for you. If you are doing something that requires more tricky 
computations on algebraic numbers, then you need the traits that you are 
currently using. 

Best,
--
Monique Teillaud 
https://members.loria.fr/Monique.Teillaud/
INRIA Nancy - Grand Est, LORIA 
Institut National de Recherche en Informatique et Automatique