Created attachment 168699 [details]
Comparison of the blur effect between KDE 5.27.10 and KDE 6.0.4

SUMMARY
Starting with KDE Plasma 6, the blur effect has noticeable artifacts when rendering at lower blur strengths, which did not happen before in KDE Plasma 5. They're best seen when using a transparent and blurred window decoration theme, such as "Blur-Glassy-Aurorae-6". 

These artifacts are present when setting the blur strength to either 0 or 1, and the artifacts differ depending on the blur strength as well. When set to 0, if a blurred window is moved around, the blur appears to jitter and appear blocky. When set to 1, detailed UI elements such as icons and especially text get blurred strangely. 

STEPS TO REPRODUCE
1. Apply a transparent window decoration theme such as "Blur-Glassy-Aurorae-6".
2. Enable blur.
3. Set blur strength to 0 or 1.
4. Move the window behind some medium to large text, and/or begin moving the window around.

OBSERVED RESULT
Blur gets rendered incorrectly with artifacts. Moving the window around produces jitter on the lowest blur strength, while when strength is set to 1, blur is rendered in a strange manner, e.g black text suddenly looks like white text with a drop shadow. See the attachments 

EXPECTED RESULT
Blur gets rendered correctly, without any jitters or strange artifacts.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux
KDE Plasma Version: 6.0.4
KDE Frameworks Version: 6.1.0
Qt Version: 6.7.0

ADDITIONAL INFORMATION

This has been reproduced on Intel integrated graphics and on a VMWare virtual machine, both on X11 and Wayland, all producing the same results, implying that this seems to be an error in the implementation of the blur effect rather than a hardware specific issue.