[Development] Why on Linux using FreeType engine fonts are rendered in 72 dpi instead of Screen DPI?

[Tomasz Olszak]

Could you please try example I sent on Linux and explain why font size
doesn't match rectangle size? Maybe the ft font engine has bug?

śr., 17 paź 2018, 19:07 użytkownik Allan Sandfeld Jensen <kde at carewolf.com>
napisał:

> Hi
>
> That is a incorrect assesment. Fonts are rendered in pixel sizes. When you
> request a QFont at a certain pixel size you get the font in that size. It
> should be noted that those sizes ONLY matter for things like pixel hinting.
> The font will typically be rendered in the same size it was requested, but
> in
> some cases a specific font-size is used at a different resolution (for
> instance with hidpi modes), in that case pixel-hinting is disable, and we
> do
> vector based rendering of the glyphs.
>
> In any case the DPI is irrelevant and only matters if you select fonts by
> point instead of pixels, and then the translation to pixels is done before
> we
> get anywhere near the font engine.
>
> 'Allan
>
> On Mittwoch, 17. Oktober 2018 17:59:26 CEST Tomasz Olszak wrote:
> > Hi,
> >
> > I would like just to ensure that I missed something and there is
> > documentation somewhere informing that Linux fonts are rendered
> > *always* in 72 dpi
> > (
> https://www.freetype.org/freetype2/docs/reference/ft2-base_interface.html#F
> > T_Set_Char_Size) (which makes font.pixelSize inaccurate for regular
> > screens).
> >
> > I found: https://bugreports.qt.io/browse/QTBUG-8890 which fixes the
> > problem but it was rejected. I see the default 72 dpi is also used in
> > current Qt 5.11 FT engine
> > (
> https://code.woboq.org/qt5/qtbase/src/platformsupport/fontdatabases/freetyp
> > e/qfontengine_ft.cpp.html#297).
> >
> > Does it surprise any of Qt developers or is it only me? Am I right
> > thinking that when we do following:
> > Rectangle {
> >    height: 100
> >    Text {font.pixelSize: 100}
> > }
> > then the relation between height of text and height of rectangle will
> > differ depending on screen dpi (don't have a way to test it
> > currently)?
> >
> > Moreover I'm pretty sure that's the cause I very often use
> > Text.fontSizeMode: Text.Fit  because setting pixelSize to height of
> > desired text size never worked correctly :)
> >
> > tl;dr: Is it a bug?
> > _______________________________________________
> > Development mailing list
> > Development at qt-project.org
> > http://lists.qt-project.org/mailman/listinfo/development
>
>
>
>
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