Actually for the chi2 kernel 2xy/(x+y) you can do a prolongation by
continuity at 0:
lim_(x->0) chi2(x,0) = 0
And you should also do it for the feature map so phi(0) = 0.
so a simple test for (x==0) will keep the feature map
correct and keep the sparsity
2012/5/30 Andreas Mueller <amuel...@ais.uni-bonn.de>
> Hi Philipp.
> The problem with using sparse matrices is that adding an epsilon
> would make them dense. I haven't really looked at it but I think
> it should somehow be possible to use this approximation also
> on sparse matrices.
> Cheers,
> Andy
>
> Am 30.05.2012 15:45, schrieb Philipp Singer:
> > Hey Andy!
> >
> > Yep I am using it successfully ;)
> >
> > The idea with adding epsilon sounds legit. I will try it definitely out.
> >
> > I think it would be nice if you could add it to your code. Would make it
> > also easier to work with sparse matrix.
> >
> > Regards,
> > Philipp
> >
> >> Hi Philipp.
> >> Great to hear that someone is using that :)
> >>
> >> The problem is that the approximation uses a "log".
> >> Afaik even the exact kernel is not defined if two features are compared
> >> that are both exactly zeros.
> >> Usually I just work around that by adding an epsilon.
> >> I was considering adding that to the code. What do you think?
> >>
> >> Cheers,
> >> Andy
> >
> >
> >
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