More consistent font rendering in Plasma

There is one thing that annoys me a bit and that’s inconsistent font rendering in Qt and GTK applications.

kate (Qt)
gedit (GTK)

The most distinctive characteristic of font rendering in Qt applications is that glyphs look thicker. Some people may argue that macOS-style font rendering is the worst one but after using Plasma for a long time, I’m used to that style of font rendering and would like fonts to look the same regardless of the underlying toolkit.

After digging though some code, I’ve discovered that Qt enables stem darkening by default in its freetype font engine. With stem darkening, glyphs are embolden to improve readability. And, indeed, after putting export FREETYPE_PROPERTIES="cff:no-stem-darkening=0" in my profile scripts, the glyphs look a bit thicker in non-Qt applications.

Note that fonts can still look differently regardless of whether stem darkening is enabled. For example, text must be rendered with linear alpha blending and gamma correction and not all toolkits do that properly.

To wrap this up, I ran Visual Studio Code with and without stem darkening to see if it makes any difference.

Visual Studio Code (Skia) w/o stem darkening
Visual Studio Code (Skia) w/ stem darkening

At quick glance, both screenshots look the same. However, after taking a closer look, you can notice that glyphs in the second screenshot are more brighter and thicker than in the first screenshot (just like how it would look in a Qt application).


The fact that Qt enables stem darkening regardless of user preferences caught me by surprise. Relying fully on system and user preferences would minimize inconsistencies and give the user more control over their machine. Either way, if you happen to use regularly applications that are built using Qt and GTK or any other toolkit, enabling stem darkening by setting the FREETYPE_PROPERTIES="cff:no-stem-darkening=0" environment variable is a good way to achieve slightly more consistent font rendering. Note that there can be inconsistencies even if all applications use the same freetype options because it still matters how toolkits perform alpha blending, etc.