Fmp4

Play a video conversion while ffmpeg is still writing it.

ffmpeg can copy a release's video into one fragmented MP4 that grows as it runs (-movflags frag_keyframe+empty_moov+default_base_moof): a fraction of a core, no segment files, and the only form of HEVC Safari plays through HLS. This package reads that file's shape — the fragments on disk, their times and byte ranges — turns it into an HLS playlist of byte ranges, and serves the file to a <video> element with range requests that wait for the writer. Nothing is decoded.

def deps do
  [{:fmp4, "~> 0.1"}]
end

Reading the file

{:ok, info} = Fmp4.scan("media.mp4")
info.init        #=> 1255          bytes of header (ftyp + moov)
info.start       #=> 0.0           seconds: where in the film the file begins
info.fragments   #=> [%{offset: 1255, size: 20569, time: 0.0, duration: 2.028}, …]

Fmp4.reach(info)     #=> 6.13      seconds of film written so far
Fmp4.bitrate(info)   #=> 86_000    bits per second, for a master playlist's BANDWIDTH

Only complete fragments are returned, so a scan during a conversion is always playable. Before the header is on disk it is {:error, :no_header}.

A file begun part-way through a longer one (ffmpeg -ss 1800 -copyts -avoid_negative_ts disabled -movflags +frag_discont) keeps the film's own clock: start is 1800 and the fragments' times count from there, so a player can seek in the whole film across several such files.

The playlist

Fmp4.playlist(info, "media.mp4", done: ffmpeg_finished?)

#EXTM3U
#EXT-X-VERSION:7
#EXT-X-TARGETDURATION:10
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-PLAYLIST-TYPE:EVENT
#EXT-X-INDEPENDENT-SEGMENTS
#EXT-X-MAP:URI="media.mp4",BYTERANGE="1255@0"
#EXTINF:2.028,
#EXT-X-BYTERANGE:20569@1255
media.mp4
…

Serve it fresh on every request: while the file grows the playlist is EVENT type and the player keeps reloading it; done: true ends it. Safari plays it natively; hls.js needs liveDurationInfinity: false to show a seek bar for an EVENT playlist.

Serving the file

def media(conn, %{"name" => name}) do
  Fmp4.RangeFile.send(conn, path(name), "video/mp4", growing?: not finished?(name))
end

Answers Range requests with 206 and Content-Range. A growing file has no total yet: its ranges say *, come a piece at a time (so the browser asks again as it fills), and a request past what exists waits up to :wait ms for the writer before 416. It works for any file — a plain MP4, a Matroska release — not only fragmented ones.

Making the file

ffmpeg -i release.mkv -c:v copy -c:a aac \
  -movflags frag_keyframe+empty_moov+default_base_moof \
  -frag_duration 2000000 media.mp4

License

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