[Interest] Is it a LGPLv3 infringement to copy paste qFuzzyCompare implementation

[Jean-Michaël Celerier]

> I was pointed out that there's a potential LGPLv3 license
infringement, although I understand where the FUD is coming from, I
think that it is OK to read LGPLv3 source code to get inspired as long
as you don't blindly copy/paste code (In my case i checked out quite a
few articles and forum threads before choosing the Qt way as the best
way to do it for my application).

There is no infrigement, you can even use directly qFuzzyCompare in your
proprietary code :
>From the LGPLv3.0

The object code form of an Application may incorporate material from a
header file that is part of the Library. You may convey such object code
under terms of your choice, provided that, if the incorporated material is
not limited to numerical parameters, data structure layouts and accessors,
or small macros, inline functions and templates (ten or fewer lines in
length), you do both of the following:

   - a) Give prominent notice with each copy of the object code that the
   Library is used in it and that the Library and its use are covered by this
   License.
   - b) Accompany the object code with a copy of the GNU GPL and this
   license document.

qFuzzyCompare clearly is in the "inline function ten or fewer lines in
length" case so you can use it without problems and without even requiring
to "give prominent notice".

Best

Jean-Michaël

On Fri, Jul 22, 2016 at 10:10 AM, Benjamin TERRIER <b.terrier at gmail.com>
wrote:

> Hi,
>
> My personal experience is that it will depend on the countries in
> which you develop
> and distribute your software.
>
> In France for instance software are protected by "author's rights" which
> implies
> that to be protected a piece of code must have some kind of creativity
> and/or
> originality. So my guess would be that according to French laws
> qFuzzyCompare
> cannot be protected and thus can be copy-pasted without infringing
> neither the law
> nor the LGPL.
> Note that this is only a guess as only judges can rule if a piece of
> code is original
> enough to be protected.
>
> If you want to be sure your best move is to contact the FSF or a
> lawyer specializing in copyright.
>
> BR,
>
> Benjamin
>
> 2016-07-22 5:00 GMT+02:00 Ch'Gans <chgans at gna.org>:
> > Hi there,
> >
> > I've recently added a "bool FuzzyCompare(float f1, float f2)" function
> > to a proprietary software I'm currently paid to work on (this is plain
> > C++ project, it doesn't use the Qt framework at all).
> > Being a more-than-happy Qt user in my spare time I decided that I
> > would first checkout the qFuzzyCompare implementation since I consider
> > Qt developers as top-of-the-notch C++ wizards (no kidding and no
> > attempt to be nice with them).
> >
> > I have added a comment to "my" implementation referring to the
> > implementation of "qFuzzyCompare" in Qt5 source code (along with [1]).
> > The 2 implementations look alike indeed (apart from using std::fabsf,
> > std::min, etc.. in place of qAbs, qMin, ...), since the fuzzy compare
> > is a well-known algorithm.
> > My reference to Qt5 was there to justify the use of the "magic
> > constant" 100000.f.
> >
> > I was pointed out that there's a potential LGPLv3 license
> > infringement, although I understand where the FUD is coming from, I
> > think that it is OK to read LGPLv3 source code to get inspired as long
> > as you don't blindly copy/paste code (In my case i checked out quite a
> > few articles and forum threads before choosing the Qt way as the best
> > way to do it for my application).
> >
> > So my question is actually very serious, in this particular case,
> > would this be considered as a LGPLv3 infringement of Qt5 to add this
> > to a piece of proprietary software:
> >    // Taken from Qt5 source code, see
> > http://doc.qt.io/qt-5/qtglobal.html#qFuzzyCompare-1 and
> >     //
> https://code.woboq.org/qt5/qtbase/src/corelib/global/qglobal.h.html#_ZL13qFuzzyCompareff
> >     // And for the insomniacs:
> >
> https://randomascii.wordpress.com/2012/02/25/comparing-floating-point-numbers-2012-edition/
> >     bool FuzzyCompare(float f1, float f2)
> >     {
> >         return (::fabsf(f1 - f2) * 100000.0f <= std::min(::fabsf(f1),
> > ::fabsf(f2)));
> >     }
> >
> > For comparison purpose, here is the Qt source code:
> > Q_DECL_CONSTEXPR static inline bool qFuzzyCompare(float p1, float p2)
> > {
> >   return (qAbs(p1 - p2) * 100000.f <= qMin(qAbs(p1), qAbs(p2)));
> > }
> >
> > You can find many more implementations of this algorithm on
> > stack-overflow and other forums.
> >
> > At the end, I proposed to change the comments to refer to wikipedia
> > instead of Qt5 source code and saying that 100000.0f is the right
> > value for epsilon.
> >
> > I know this is a drop in an ocean (we're talking 1 line of C++), but i
> > think the question is indeed interesting. Thanks for shedding light on
> > this particular issue.
> >
> > Chris
> >
> >
> > [1]
> https://randomascii.wordpress.com/2012/02/25/comparing-floating-point-numbers-2012-edition/
> > _______________________________________________
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> > Interest at qt-project.org
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