Fractional display scaling on X11

X11 supports fractional scaling perfectly well, but most GUI toolkits don’t implement it properly. Scaling works fine in a pure QT environment, but when using other GUI toolkits (such as GTK+), things don’t work as expected due to them being poorly written. This article describes several methods of universal mixed display scaling with broken GUI toolkits and a simple window manager (dwm).

A huge chunk of this article is inspired by William Storey and Rico Sta. Cruz (see Resources).

How scaling works

There is a more detailed explaination written by Giuseppe Bilotta in his article.

In short: Scaling support has to be implemented properly by the application, or subsequently the GUI toolkit. Most toolkits and applications are broken and fractional scaling has to be hacked together to be universal.

There are several ways to set UI scaling and each works for different things. They need to be combined to affect most of the system and they can either affect font size or UI elements, such as margins, paddings, etc. The intended X11 way is setting Xft.dpi in .Xresources. Some toolkits do not support this and instead use custom environment variables.

Calculating DPI

The DPI can either be calculated or obtained for a display datasheet. Scaled DPI can be calculated by subtracting n% to get the desired value. For instance, my display has 277 DPI and my desired scaling is 150%.

277 - 50% = 138.5

Even though any scaling value can be set, it is best to select one of the following values closest to the calculation result. The reason for this is that arbitrary fractions may result in broken bitmap scaling.


Scale:	100%	125%	150%	175%	200%
DPI:	96 (default)	120	144	168	192

In my particular case, the most optimal DPI value would be 144.

Simple UI scaling


Multiple mixed-DPI monitors	No
Requires xserver restart	Yes
Performance	Fast
Image Quality	Good

This allows us to scale the UI by any factor, but it will not work with mixed-DPI monitors. Switching to different scaling also requires an xserver restart, or restarting all individual applications.

More information about different toolkits can be found at Archlinux Wiki.

Xft.dpi

Global xrandr configuration for UI scaling is set in .Xresources by setting Xft.dpi to the desired value, eg.

Xft.dpi: 144

QT

QT scaling on legacy applications can be enabled by exporting QT_AUTO_SCREEN_SCALE_FACTOR=1.

In case the core protocol does not report our primary monitor DPI correctly, we have to set the DPI manually, eg. in our .profile or .bash_profile.

export QT_AUTO_SCREEN_SCALE_FACTOR=0
export QT_SCALE_FACTOR=1.5
export QT_FONT_DPI=96

QT_AUTO_SCREEN_SCALE_FACTOR=0 disables the automatic scaling, as some applications with forces dcaling would get scaled twice. QT_SCALE_FACTOR=1.5 sets the desired scale factor (150%). Since we have set xrdb DPI to support other toolkits, this alo will result in huge fonts, so they need to be reset back to default by QT_FONT_DPI=96. This should result in good-looking QT scaling.

GDK3

Since we are using a simple window manager, the Xft.dpi setting should scale everything properly.

In case your icons don’t scale well, you can force GDK scaling with GDK_SCALE and GDK_DPI_SCALE.

export GDK_SCALE=2
export GDK_DPI_SCALE=0.5

This would scale everything up by the factor of 2 (it only accepts an integer) and scale fonts back down by 50%. Despite fonts being scaled properly, the resulting margins/paddings and icons are larger, since they get scaled by 200%, not 150%.

Mixed monitors with different DPI


Multiple mixed-DPI monitors	Yes
Requires xserver restart	No
Performance	Poor
Image Quality	Poor on low-DPI displays

In case you want multiple monitors with mixed DPI, this method has to be used. Despite working as intended, the downscaling results in a poorer image quality on low-DPI displays. Large scaling factors may also result in poor performance. In my case, browser lag while scolling was very noticeable.

First step is to scale the UI by 200%, similar to the previous step.

Add the following to .Xresources:

Xft.dpi: 192

and to .bash_profile:

export GDK_SCALE=2
export GDK_DPI_SCALE=0.5
export QT_AUTO_SCREEN_SCALE_FACTOR=0
export QT_SCALE_FACTOR=2
export QT_FONT_DPI=96

Next step is to configure downscaling in xrandr by a specific factor for each monitor.

...todo...

Changing the scaling factor can be done simply by running xrandr again with chosen scaling factor and everything works as it should.

Citing Giuseppe Bilotta:

If you think this idea is a bit stupid, shed a tear for the future of the display servers: this same mechanism is essentially how Wayland compositors – Wayland being the purported future replacement for X – cope with mixed-DPI setups.

Resources

2021-12-27, Emil Miler

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