CGAL users discussion list

[Jim Wilenius <>]

Hi!

Thanks for replying! =)

I would very much appreciate if you could explain in more detail...

I don't understand why the kernel must be exact, even if new points are 
created. Is this a specifically introduced limitation in CGAL or is it 
that the use of inexact kernel causes errors due to floating point 
representation which makes the algorithm throw exceptions.

In my specific case, I would be satisfied if the precision of the 
Point_2 is around 1E-9 which is very "dull" compared to the smallest 
double. Is it possible to do this in CGAL? or do I need a new type of 
Kernel. What I mean is, in my case, I can consider values to be the same 
if the difference is "small". Is there a Kernel for this? 
"epsilon_Kernel" =)

I realize I should read more about Kernels... but in the mean time... =)

Regards,
Jim


Efraim Fogel wrote:
> During the computation of Minkowski sums new points might be
> constructed. These points are used as operands of predicates. Thus, you
> need an exact-construction kernel (as well as an exact-predicate
> kernel). I didn't understand the second question.
>
> jim.wilenius wrote:
> > Hi!
> >  
> >  I am using the CGAL lib in OSX 10.6. (freshly compiled, I followed the
> > instructions carefully)
> >
> > I am using the CGAL::Exact_predicates_inexact_constructions_kernel and when
> > running minkowski sum I get:
> > ---
> > terminate called after throwing an instance of
> > 'CGAL::Precondition_exception'
> >   what():  CGAL ERROR: precondition violation!
> > Expr: cv.is_in_x_range (p)
> > File: /usr/local/include/CGAL/Arr_segment_traits_2.h
> > Line: 486
> > ---
> >
> > I get the error given the following two polygons (in order):
> >
> > P:
> > 4
> > 0.0 0.0
> > 29.809108311772626 0.0
> > 29.809108311772626 29.809108311772626
> > 0.0 29.809108311772626
> >
> > Q:
> > 7
> > 99.10004234616522 202.2922116762398
> > 168.16528534816382 202.2922116762398
> > 168.16528534816382 271.3574546782384
> > 154.3522367477641 271.3574546782384
> > 140.53918814736437 243.73135747743896
> > 126.72613954696467 229.91830887703924
> > 99.10004234616522 216.1052602766395
> >
> > I am not sure exactly what the error means? Is it supposed to give an error?
> > If I use CGAL::Exact_predicates_exact_constructions_kernel then I do not get
> > the error.
> >
> > using exact constructions is extremely slow for my application, so I would
> > really like to use inexact.
> >
> > by the way, how inexact is the inexact kernel? Any pointers to useful info
> > is welcome.
> >
> > Regards,
> > Jim
> >   

--
Jim Wilenius