shundhammer/y-qt-hdpi-and-fonts.md

## y-qt-hdpi-and-fonts.md

      
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              y-qt-hdpi-and-fonts.md
            
          
    YaST Qt, HiDPI and Fonts

Links

Bugzilla


https://bugzilla.suse.com/show_bug.cgi?id=1199020

Web


Qt ref doc about HiDPI: https://doc-snapshots.qt.io/qt5-5.15/highdpi.html

Latest News

We are now setting those environment variables directly:
QT_SCALE_FACTOR=1.75

(for 168 dpi)
QT_SCALE_FACTOR_ROUNDING_POLICY="PassThrough"

(to avoid rounding)
We are no longer setting the Xft.dpi X resource.
If the monitor reports no reasonable EDID, boot the installation ISO with your real monitor width in millimeters (use an old-fashioned ruler to measure it):
YAST_MON_WIDTH_MM=260

More details: yast/yast-installation#1057
Older Notes

Source Code


yast2 start scripts:

yast2 script: https://github.com/yast/yast-yast2/blob/master/scripts/yast2
YaST2.call: https://github.com/yast/yast-installation/blob/master/startup/YaST2.call


yast-x11 tools:


xftdpi.c: https://github.com/yast/yast-x11/blob/master/src/tools/xftdpi.c
(lightweight way to set Xft.dpi instead of full-blown xrdb)


libyui-qt

YQUI.cc: https://github.com/libyui/libyui/blob/master/libyui-qt/src/YQUI.cc

YQUI::calcDefaultSize()
YQUI: Setting Qt HiDPI mode: https://github.com/libyui/libyui/blob/master/libyui-qt/src/YQUI.cc#L159
QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);


YQApplication.cc

YQApplication::pickAutoFonts()


Qt

class QHighDpiScaling in src/gui/kernel/:

https://github.com/qt/qtbase/blob/5.15.2/src/gui/kernel/qhighdpiscaling_p.h
https://github.com/qt/qtbase/blob/5.15.2/src/gui/kernel/qhighdpiscaling.cpp


Handling the Xft.dpi X resource:

Reading the X resource: QXcbVirtualDesktop::readXResources()
QXcbScreen::forcedDpi()
QXcbScreen::logicalDpi()


QGuiApplication initializes its highDpiScaleFactorRoundingPolicy to Qt::HighDpiScaleFactorRoundingPolicy::Round, so the scale factor will always be 1.0 or 2.0, never something like 1.5.


linuxrc

PR: Add EDID boot option: openSUSE/linuxrc#297


Qt HiDPI Interfaces

Related Qt Environment Variables


QT_SCALE_FACTOR
QT_SCALE_FACTOR_ROUNDING_POLICY

Round
Ceil
Floor
RoundPreferFloor
PassThrough


QT_SCREEN_SCALE_FACTORS
QT_USE_PHYSICAL_DPI
QT_FONT_DPI
QT_DPI_ADJUSTMENT_POLICY
QT_ENABLE_HIGHDPI_SCALING


QT_AUTO_SCREEN_SCALE_FACTOR (X11 only, deprecated on all other platforms)
QT_DEVICE_PIXEL_RATIO (deprecated)

Notes

Common HiDPI Resolutions


2560x1080 UW-UXGA
2560x1440 WQHD, 1440p
2560x1600 WQXGA
3200x1600 QHD+
3440x1440 UW-QHD
3840x1600 UW-QHD+
3840x2160 QFHD, 4K, UltraHD, UHD-1
3840x2400 WQUXGA
4096x2160 DCI 4K
5120x2880 5K

Common DPI Values

All multiples of 48

96 (min for YaST)
144
192
240
288 (max for YaST)
336
384

Notes from Digging in the Qt Sources


We set Qt::AA_DisableHighDpiScaling,
so in gui/kernel/qhighdpiscaling.cpp

QHighDpiScaling::m_usePixelDensity is true,
usePixelDensity() returns true
QHighDpiScaling::m_pixelDensityScalingActive is true,
QHighDpiScaling::m_active is true


By default, Qt rounds the scale factor to an integer;
so in practice, it's either 1.0 or 2.0.


We set the Xft.dpi X resource,
so in plugins/platforms/xcb/qxcbscreen.cpp
the value is picked up in QXcbVirtualDesktop::readXResources(), so it is returned by

QXcbVirtualDesktop::m_forcedDpi
QXcbVirtualDesktop::forcedDpi()
QXcbScreen::forcedDpi()
QXcbScreen::logicalDpi()

and QWindowSystemInterface::handleScreenLogicalDotsPerInchChange() is called with that value.


logicalBaseDpi() has a default implementation in gui/kernel/qplatformscreen.h as QDpi( 96, 96 ), and the xcb platform (X11) overrides it with the same value in platforms/xcb/qxcbscreen.h.


The ratio between that logicalBaseDpi (a default value of 96 dpi) and the logicalDpi() value (i.e. what we set in the Xft.dpi X resource based on what xrandr tells us) is the raw scale factor:
factor = qreal(platformLogicalDpi.first) / qreal(platformBaseDpi.first);
e.g. 144 / 96 = 1.5
which is then rounded to the next integer value, i.e. 2 in this case.


y2logs

y2logs Tarball from Comment #2 (jsudermann)

Xorg.0.log

[    82.995] (II) modeset(0): clock: 152.8 MHz   Image Size:  344 x 193 mm
[    82.995] (II) modeset(0): h_active: 1920  h_sync: 2000  h_sync_end 2054 h_blank_end 2250 h_border: 0
[    82.995] (II) modeset(0): v_active: 1080  v_sync: 1086  v_sync_end 1094 v_blanking: 1132 v_border: 0

y2start.log

Stage [call]: Monitor size: eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 344mm x 193mm
Stage [call]: Monitor width mm: 344
Stage [call]: Monitor width px: 1920
Stage [call]: Monitor dpi: 144
Stage [call]: Xft.dpi set to: 144

y2log

2022-04-29 14:43:46 <1> install(4700) [qt-ui]

  YQUI.cc(calcDefaultSize):370 -fullscreen: using 960 x 540 for `opt(`defaultsize)
  YQUI.cc(calcDefaultSize):404 Default size: 960 x 540
  YQApplication.cc(pickAutoFonts):471 Selecting auto fonts - normal: 10, heading: 12 (bold)
  YQApplication.cc(currentFont):339 Loaded 10 pixel font: Sans Serif,-1,10,5,50,0,0,0,0,0

YQUI.cc(calcDefaultSize):370 -fullscreen: using 960 x 540 for `opt(`defaultsize)

?!? Why does it use 960 x 540 for fullscreen?
https://github.com/libyui/libyui/blob/master/libyui-qt/src/YQUI.cc#L370
    QSize availableSize = screen->availableSize();
So Qt thinks we are in HiDPI mode, scaling everything? It's using half the resolution values of 1920 x 1080 in both dimensions.
And we also requested only a moderate font size: 10 px for normal text, 12 px for headings.