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The Raspberry Pi 4 can encode videos using hardware acceleration by using 64 bit Raspberry Pi OS, a particular ffmpeg fork and the h264_v4l2m2m codec. Alternatively, Ubuntu and the 4.4 version of ffmpeg available via apt also yields the same results (but runs at much lower fps). For example:

ffmpeg -i big_buck_bunny_720p_10mb.mp4 -c:v h264_v4l2m2m -c:a copy out.mp4

However, ffmpeg will emit one warning:

Failed to set gop size: Invalid argument

This means that the -g option, and hence information about number/ distance of keyframes, can't be set. Example for setting gop explicitly, which will result in the same warning:

ffmpeg -f v4l2 -s 1280x720 -r 25 -i /dev/video0 -c:v h264_v4l2m2m -g 50 -keyint_min 25 -c:a copy out.mp4

This doesn't seem to be much of a problem when encoding to a video file, which works fine, but makes it impossible to stream the video data to services like YouTube or Twitch.

Using the non-hardware accelerated libx264 instead will enable to set the GOP/ keyframes, and enable streaming to aforementioned platforms. However, without acceleration, the performance makes this practically impossible.

I'm not sure whether this is a problem with the h264_v4l2m2m codec, this ffmepg fork (I've opened an issue report), ffmpeg in general, drivers or something else. I am therefore looking for insights into what causes this and how to either work around or - ideally - solve this issue. In short, how to use hardware acceleration, yet set the GOP/ keyframes?

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