Re: [Lib2geom-devel] size comparison in Affine::inverse()

[Nathan H. <nj...@nj...>]

Oh right, yeah, Alvin's absolutely(relatively?) right.  You get a
ship-it/lgtm from me.

njh

On Mon, Jul 21, 2014 at 11:22:49PM +0200, Johan Engelen wrote:
> Hi Nathan, Alvin,
>    I was also confused a bit at first, but:
> inline bool rel_error_bound(Coord a, Coord b, double eps=EPSILON) { return a <= eps*b && a >= -eps*b; }
> 
> So the fix with sqrt does do a different check.
> 
> hth
> regards,
>    Johan
> 
> On 21-7-2014 23:08, Nathan Hurst wrote:
> > Does it fix the bug?  I don't understand because those two algorithms
> > are almost equivalent (there is a slight shift in the bounds due to
> > nonlinearity).  In this case det should be 0.150, which is a long way
> > from singular.  What is mx in this case?
> >
> > njh
> >
> > On Mon, Jul 21, 2014 at 03:56:48PM -0400, Alvin Penner wrote:
> >> while investigating LP Bug 1231990,
> >> https://bugs.launchpad.net/inkscape/+bug/1231990, it was found that
> >> the routine Affine::inverse() in the file affine.cpp is apparently
> >> refusing to calculate the inverse of the following matrix:
> >>
> >> 0.001,    0.0
> >> 0.0,    150.0
> >>
> >> The refusal is coming from the line:
> >> if (!rel_error_bound(determ, mx*mx))
> >>
> >> I would like to propose that this be changed to:
> >> if (!rel_error_bound(std::sqrt(fabs(determ)), mx))
> >> so that we would be comparing two numbers which both have units of
> >> length. This would be similar to a change was made in rev 2183, where
> >> a similar size comparison problem was encountered (LP Bug 1309225).
> >>
> >> Any comments?
> >>
> >> Cheers,
> >> Alvin
> >>
> >>
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