On Thu, November 29, 2012 14:53:21 Sean Harmer wrote: > On Thursday 29 November 2012 15:01:05 Dominik Holland wrote: > > Hi all, > > > > last month i had some performance problems on a QML animation running on > > low end hardware (imx233). > > I debugged that problem and it was caused because of rounding errors > > during the comparison of two reals. > > > > I fixed that problem by replacing the comparison by a qFuzzyCompare() in > > the Qt4 declarative source code. > > Afterwards i checked whether qt5 has the same problem... and it has > > exactly the same problem. > > > > Now i pushed my patch to gerrit > > (https://codereview.qt-project.org/#change,40655) and looked again at > > the code > > for any other occurrences. > > > > What i found is, that this happens in many places and it would be a > > really huge effort to change the code to compare the qreals in the right > > way. > > > > Is this already a known problem and do you think that it is worth to try > > to fix the code ? > > Also please be aware that qFuzzyCompare() is nto the right way to compare > two arbitrary floating point values. If one of them is zero or close to > zero the condition it tests for becomes impossible to satisfy. > > Something along the lines of > > fuzzyCompare(float p1, float p2) > { > if (qFuzzyIsNull(p1)) > return qFuzzyIsNull(p2); > else if (qFuzzyIsNull(p2)) > return false; > else > return qFuzzyCompare(p1, p2); > } > > is slightly better but still not fantastic. Comparing two floats that we do > not know a priori is actually very difficult > > http://randomascii.wordpress.com/2012/02/25/comparing-floating-point- > numbers-2012-edition/ > > I would love to see a better qFuzzyCompare() implementation in Qt but the > existing one is used in so many places it would be a nightmare to introduce > without unwanted side effects.
I think it's not about mathematical correctness. It's about deciding if something like width or height is updated or not. So for those use-cases it's IMO enough to filter a lot/nearly all unnecessary updates out. > > Cheers, > > Sean > > -- > Dr Sean Harmer | sean.har...@kdab.com | Senior Software Engineer > Klarälvdalens Datakonsult AB, a KDAB Group company > Tel. Sweden (HQ) +46-563-540090, USA +1-866-777-KDAB(5322) > KDAB - Qt Experts - Platform-independent software solutions > _______________________________________________ > Development mailing list > Development@qt-project.org > http://lists.qt-project.org/mailman/listinfo/development _______________________________________________ Development mailing list Development@qt-project.org http://lists.qt-project.org/mailman/listinfo/development
