QtMultimedia 6.6.0 would select dynamic VAAPI on linux, then warns
during build (even though it chose this on purpose):
> QT_FEATURE_vaapi is found but ffmpeg doesn't include vaapi,
> however dynamic symbols resolve is possible
The nuisance warning was fixed for 6.7 and backported to 6.6.1:
https://codereview.qt-project.org/c/qt/qtmultimedia/+/517333
However, tracing it helped me figure out why vaapi actually wasn't
working: nix doesn't end up with an rpath such that dlopen("va")
can actually find libva.so in the nix store, thus failing at runtime:
> qt.multimedia.plugin: loading backend "ffmpeg"
> qt.core.library: "/nix/store/i9fkjks6dfjj1p9qvj5633sxbrf5rbd8-qtmultimedia-6.6.1/lib/qt-6/plugins/multimedia/libffmpegmediaplugin.so" loaded library
> qt.multimedia.ffmpeg.libsymbolsresolver: Start VAAPI symbols resolving: 39 symbols
> qt.core.library: "va" cannot load: Cannot load library va: (va: cannot open shared object file: No such file or directory)
> qt.multimedia.ffmpeg.libsymbolsresolver: Couldn't load VAAPI library
7e886531a2