CGAL users discussion list

[Ben Supnik <>]

Hi Y'all,

Thanks - it does look like the sqrt function is used only to construct 
splits, and I can probably survive with inexactness if my input isn't 
too horribly degenerate. :-)

One more question: what's the least bad way for me to collect all of the 
vertices inserted into my triangulation during the make-conforming 
operation?

I don't see any notifier interface, so it looks like my options would 
mostly involve overriding, er, something in the TDS to detect vertex 
creation.

cheers
ben


Fernando Cacciola wrote:
> Hi Ben,
> Hu Sylvain,
>
> Sylvain Pion wrote:
> > Ben Supnik wrote:
> > > The docs for make_conforming_Delaunay_2 say that it requires 
> > > ConformingDelaunayTriangulationTraits_2. traits.
> > >
> > > ConformingDelaunayTriangulationTraits_2 is labeled as having "Any 
> > > model of Kernel concept. In particular, all CGAL kernels."
> > >
> > > But...this doesn't seem correct to me - am I right in thinking that 
> > > some CGAL exact pred exact construction kernels do not have sqrt?  
> > > The one I was using did not, and I get a compile failure due to being 
> > > unable to tkae the sqrt of a number.
> > >
> > > If sqrt is indeed required and not in all kernels, is there a way to 
> > > get an exact predicate/exact construction kernel without one of LEDA 
> > > or GMP?
> >
> > Known issue (to me at least).
> >
> > One way to cheat is to define a sqrt() function for the number type
> > that you use, and make it approximate (convert to double, use std::sqrt,
> > and convert back).  It might work for some/most rational number types,
> > and hopefully the algorithms won't break too much...
>
> FWIW, the straight skeleton package uses such a cheat.
>
> If you look at the header:
>
> CGAL/constructions/Straight_skeleton_cons_ftC2.h
>
> There is the function:
>
> inexact_sqrt(num)
>
> which does what Sylvain said.
>
> You might wan't to base your version on that one.
>
> It works for arbitrary number types, it uses CORE::BigFloat instead of 
> double if available, and if it uses double, it checks for overflow after 
> the to_double conversion, and for Quotient<NT> it avoids the division.
>
>
> Best
>
> Fernando Cacciola
> www.geometryfactory.com

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