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

[Tomasz Olszak]

If I understand correctly font.pixelSize is actually not the same as
pixels user for with and height of items. So if you render rectangle
of height e.g 100 pixels on 96 dpi screen, and render Text with
font.pixelSize = 100 on 96 screen the iheight of text will be exactly
100 * 72/96 which is exactly 75% of rectangle's height. If you render
the same example on screen with 72 dpi they will be exactly the same,
on 192 dpi text will be ~37% of rectangle's height. It is just a
guess, all my 4 screens I have currently access to are 96 dpi. It is
just a guess and if Qt doesn't do something under the hood, FT engine
renders fonts in 72 dpi which means that actually on screen text with
a font with 100 pixels size is smaller than item with 100 pixel size .
That's why I'm asking here not on Interest.

Device pixel ratio can be used to calculate screen DPI in Qt Quick - I
use it now to correctly render fonts using pixelSize (fontPixelSize
factor == Screen.dpi / 72). Pixel size is given by ocring image in 300
dpi. But pixel is pixel and it should work. Text however always was a
bit smaller. When I calculated pointSize then the text was correct. I
dug more and ended up with conclusions as described above.

śr., 17 paź 2018 o 18:21 Jason H <jhihn at gmx.com> napisał(a):
>
> A pixel is a pixel, regardless of DPI.
>
> However things matter when you use point sizes (which assumes 72 points per inch) there are also Screen.devicePixelRatio  ( http://doc.qt.io/qt-5/qml-qtquick-window-screen.html#devicePixelRatio-attached-prop ) to contend with. I haven't had to worry about this for some time, so my intel may be out of date. It was a mess around 5.6...
>
> I'm assuming the 72 has something to do with the point size, and being able to calculate scaling relative to 72.
>
>
> > Sent: Wednesday, October 17, 2018 at 11:59 AM
> > From: "Tomasz Olszak" <olszak.tomasz at gmail.com>
> > To: development at qt-project.org
> > Subject: [Development] Why on Linux using FreeType engine fonts are rendered in 72 dpi instead of Screen DPI?
> >
> > 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#FT_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/freetype/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?
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> >