[Development] FP calculations and stability in Qt

[NIkolai Marchenko]

>  I ended up using "proper" geometry processing library for the "model"
and used Qt to do the rendering (the view).

Somehow I get the feeling you just saved me a ton of headaches in the
future :) thx

On Sun, May 12, 2019 at 1:50 PM Christian Gagneraud <chgans at gmail.com>
wrote:

> On Sun, 12 May 2019 at 20:26, Konstantin Shegunov <kshegunov at gmail.com>
> wrote:
> >
> > Hi,
> > I'd want to clear the context out of the way, so this is the bug[1] that
> got me thinking.
> > I appreciate that we want to keep external dependencies to a minimum,
> and for a good reason, but can we talk about how feasible it is to pull
> something (or parts of it) in Qt, even if only internally, to facilitate
> stable fp calculation.
> >
> > I've seen some complaints (on the forums mostly) about the stability of
> the transformations Qt supplies, generally unfounded, however it'd be nice
> if we can solve this with generality. I realize Qt is no math library and
> support the idea to drop most of the global functions that dealt with math,
> in the end they're already in the STL. However, as it is, we have some
> linear algebra done (internally mostly), so I think it'd be nice to have
> that done "correctly" for the user.
> >
>
> I use to think that Qt could do a better job about FP
> precision/stability, but i had to realise that i was using Qt in a way
> that it was not designed for.
> For example, I tried to use QPainterPath, QLineF, QRectF, ... to do
> geometry processing. And i can tell you that QPainterPath is all but
> stable when it comes to small values. Highly zoomed-in QGraphicsView
> based geometry object yields crazy artifacts.
> I then turned on specialised library, Qt is a GUI toolkit, and is
> optimised for painting. Qt takes all opportunities to be fast and
> efficient in that context, and that includes being "mathematically"
> imprecised, painting is all about pixels at the end of the day.
> Who cares where exactly a line intersect a polygon, if all you need to
> know is if you need to paint a pixel black or red.
>
> To summarise:
> I ended up using "proper" geometry processing library for the "model"
> and used Qt to do the rendering (the view).
> It came at a cost, but in the long term was a big win.
>
> Chris
>
> > I'd appreciate feelings and opinions on the matter, as I'm but one
> person and my view is rather limited.
> >
> > [1]: https://bugreports.qt.io/browse/QTBUG-75146
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