Download/Import a big, high quality PNG (or pure RAW/ARW/NEF) file;
Install libavif (if you haven’t already, kimageformats should be the default in KDE);
Compress photo (for RAW files sometimes intermediary as PNG is required):
method 1:
$ avifenc -c aom -j all -s 0 -a aq-mode=2 -d 8 input.png output.avif
method 2:
$ avifenc -c aom -j all -s 0 -d 8 input.png output.avif
Can someone could confirm the bug? Do you reproduce it? Please try to use avif as locking screen background, SDDM login background and normal wallpaper. See if you can display avif in gwenview and if you are seeing thumbnails in Dolphin.
Can you please confirm that kimageformats removed support for avif compression?
I converted a PNG image using either method 1 or method 2. With the AVIF image I obtained, Dolphin is able to show the thumbnail, Gwenview is able to show the image, and I can use it as wallpaper. I didn’t try as login / locking screen background.
When I installed the kimageformats package, I also installed all the optional dependencies. Only one of them was not already installed, as shown from the yay -S kimageformats output.
(1/1) installing kimageformats
Optional dependencies for kimageformats
karchive: plugin for Krita and OpenRaster images [installed]
libavif: AVIF format support [installed]
openexr: EXR format support [installed]
libheif: HEIF format support [installed]
libjxl: JPEG-XL format support
libraw: RAW format support [installed]
As a side note, on my computer, the kimageformats package wasn’t installed. If there are issues with updating it, I would not notice them as I installed directly the latest release.