Declare media sources and select their streams.

Create an input with FFix.input/2 or new/2. A file may contain video, several audio tracks, subtitles, and other streams. Select the tracks you want before applying filters or declaring outputs.

Select tracks

source = FFix.input("interview.mp4")
picture = FFix.video(source, 0)
sound = FFix.audio(source, 0)
output = FFix.output([picture, sound], "interview.mkv")

Indexes start at zero within each media type. audio(source, 1) selects the second audio track, regardless of where video or subtitles occur in the file. FFix.select(source, 3) instead counts every stream in the file.

Use :all to retain every track of a media type. optional: true lets FFmpeg omit a missing match, which is useful for files that may have no audio:

sources = [
  FFix.stream_copy(FFix.video(source, 0)),
  FFix.stream_copy(FFix.audio(source, :all, optional: true))
]

FFix.output(sources, "interview.mkv") |> FFix.command()

An indexed, required selection can feed a filter. Broad, optional, and string selections return FFix.Selection values for output mapping. Their number of matches is determined by FFmpeg when it reads the input. Even a string like "v:0" is treated as a query; use FFix.video(source, 0) for a filter input.

To find the tracks in a file, use ffprobe, for example ffprobe -v error -show_streams interview.mp4. FFix.Discovery describes the FFmpeg installation's capabilities rather than the contents of a media file.

Reuse an input

Reusing the same declaration across outputs opens the source once. Declare the same file twice when you need different input settings:

opening = FFix.input("interview.mp4", ss: 0)
ending = FFix.input("interview.mp4", ss: 120)

outputs = [
  FFix.output(FFix.video(opening, 0), "opening.mp4", t: 10),
  FFix.output(FFix.video(ending, 0), "ending.mp4", t: 10)
]

FFix.command(outputs)

Configure seeking and FFix.Decoder options before selecting streams. A stream keeps the input configuration it was selected from, so selecting first and changing that input later creates conflicting configurations.

Input formats and options

FFmpeg usually detects file formats. Use FFix.Demuxer when a format needs explicit settings, such as raw frame dimensions. FFix.Decoder configures how individual tracks are decoded.

General input options are written before this source's -i. For example, ss: 30 seeks before decoding. See FFix.Command for option syntax and the FFmpeg input options for their meaning.