Fractional scaling support on wayland

[eddy-geek]

Hello,

gpxsee 11.12
Qt 5.15.8
Linux 5.19.0-24-generic
kwin 5.26.90

it seems there are issues with fractional support on Plasma/KWin/Wayland.

My system is set at 135% and what I think is happening is:

• "Standard" mode - ok, image are drawn at 135%
• "High-resolution" mode - I think they are drawn at 2x then scaled-down, so in the end 1 image pixels is drawn on 1.35/2 ≈ 0.67 screen pixels.

It can probably be linked to the way Qt scaling currently works on wayland (I think it's making apps draw at ceil(ratio) then the compositor downscales). Since work is ongoing to change this with this recent wayland change and Qt 5 patch (which I don't think my system has yet), if eg Linux-X11 & Windows work fine, I guess it's fine to hold off for the time it takes for the ecosystem to catch-up.

[tumic0]

The HiDPI map scaling should be fractional (but you can not get non-blurry images in other multiplications than 2^n), but I'm not sure what exactly you are referring - your image/screenshot mix is blurred as whole, not only the map. This would suggest some global scaling problem.

However, like with other exotic configurations, there is not much I can do about it until someone completely describes what is the root of the issue and how it can be solved. Without having some reference system (I don't/can't have wayland on my linux machine and I do not have a HiDPI monitor) I can hardly investigate/solve such issues.

[tumic0]

QTBUG-123125