Preparing videos for Mastodon using ffmpeg

By Danny van Kooten on Nov 2, 2023 on Permalink.

Recently I was recording a screencast of me launching Koko Analytics as a Progressive Web App from my OS launcher. Trying to upload the video to Mastodon failed with the following error message:

422 1000fps videos are not supported

According to the Mastodon documentation, the following limits are in place for video uploads:

Supported formats: MP4, M4V, MOV, WebM
Maximum size: 40MB
Maximum bitrate: 1300kbps
Maximum framerate: 60fps

The documentation does not state anything about a maximum resolution, but other sources stated a video could be no larger than 1920 x 1200 px.

Downsample video resolution, fps and bitrate using ffmpeg

ffmpeg is an incredible piece of software to convert video between various formats.

To use it to downsample a video to match the Mastodon requirements, we can use the following command:

ffmpeg                              \
  -i in.webm                        \
  -filter:v scale=1900:-1,fps=60    \
  out.webm

This resizes the video to 1900px wide while preserving the aspect ratio. It also limits the framerate to 60 fps.

If your video has a bitrate higher than 1300kbps, instruct the encoder to aim for a lower bitrate using the -b:v argument.

ffmpeg                                    \
  -i in.webm                              \
  -filter:v scale=1900:-1,fps=60          \
  -b:v 1300k -maxrate 1300k -bufsize=650k \
  out.webm

Since -b:v specifies the target (average) bitrate for the encoder to use, specifying it only makes sense if your video has a larger bitrate than what is required by Mastodon.

I had to do a bit of browsing around to find the proper set of arguments, so hopefully sharing this here helps someone shave a few minutes off their day.

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Comments are welcome. You can email me at hi @ this domain.