Also, I found this text, explaining various options you can pass to hw encoders:
https://gist.github.com/Brainiarc7/4b49f463a08377530df6cecb8171306a
FFmpeg and libav's playbook: Advanced encoding options with hardware-based acceleration, NVIDIA's NVENC and Intel's VAAPI-based encoder.
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ffmpeg -h encoder=h264_vaapi
Encoder h264_vaapi [H.264/AVC (VAAPI)]:
General capabilities: delay
Threading capabilities: none
Supported pixel formats: vaapi_vld
h264_vaapi AVOptions:
-qp <int> E..V.... Constant QP (for P-frames; scaled by qfactor/qoffset for I/B) (from 0 to 52) (default 20)
-quality <int> E..V.... Set encode quality (trades off against speed, higher is faster) (from 0 to 8) (default 0)
-low_power <int> E..V.... Use low-power encoding mode (experimental: only supported on some platforms, does not support all features) (from 0 to 1) (default 0)
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