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

[Johan E. <jbc...@sw...>]

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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