Re: [OctDev] geometry: wrong computation of axes in cov2ellipse

[Juan P. C. <aju...@gm...>]

On Sun, Nov 25, 2012 at 2:54 PM, Rafael Laboissiere
<ra...@la...> wrote:
> The function cov2ellipse in the geometry package is not computing the
> ellipse axes correctly.  The first problem is that the scale of the axes is
> wrong.  This can be demonstrated with the code below
>
>   ######################################################################
>   function cov2ellipse_demo (K)
>       L = chol(K,'lower');
>       u = randn(1e3,2)*L';
>       elli = cov2ellipse (K);
>       figure(1)
>       plot(u(:,1),u(:,2),'.r');
>       hold on;
>       drawEllipse(elli,'linewidth',2);
>       hold off;
>       axis tight;
>   endfunction
>   cov2ellipse_demo (K); ## This is what we get with "demo cov2ellipse"
>   cov2ellipse_demo (10*K);
>   cov2ellipse_demo (0.1*K);
>   ######################################################################
>
> This is fixed by taking the square root of the eigenvalues obtained by
> svd().
>
> The second problem is that the axis orientation is wrong, evidenced in the
> code below:
>
>   ######################################################################
>   K = [2 -1; -1 2];
>   cov2ellipse_demo (K)
>   ######################################################################
>
> This happens because the wrong elements of the eigenvectors are taken.
>
> The patch attached below fixes both problems.
>
> Rafael
>
>
> ------------------------------------------------------------------------------
> Monitor your physical, virtual and cloud infrastructure from a single
> web console. Get in-depth insight into apps, servers, databases, vmware,
> SAP, cloud infrastructure, etc. Download 30-day Free Trial.
> Pricing starts from $795 for 25 servers or applications!
> http://p.sf.net/sfu/zoho_dev2dev_nov
> _______________________________________________
> Octave-dev mailing list
> Oct...@li...
> https://lists.sourceforge.net/lists/listinfo/octave-dev
>

Thank you again Rafael!

I applied the patch.

How did you find this bug?