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

[Tomasz Olszak]

Sorry accidental Shift+Enter

https://pastebin.com/m2tAgT3f

For my screens QScreen::physicalDotsPerInch() return ~92dpi instead of
96 but it is probably due to some platform edid implementation or
something like that.
śr., 17 paź 2018 o 19:04 Tomasz Olszak <olszak.tomasz at gmail.com> napisał(a):
>
> Here you can find and example of what I believe is true:
>
> śr., 17 paź 2018 o 18:48 Tomasz Olszak <olszak.tomasz at gmail.com> napisał(a):
> >
> > 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 I haven't had to worry about this for some time, so my intel may be out of date.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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> > > > Development at qt-project.org
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> > > >