A query selecting input streams for an output.

Media helpers return selections for :all or optional: true. FFix.select/3 also returns them for string stream specifiers. Pass a selection to FFix.output/3, FFix.Encoder, or FFix.stream_copy/1.

FFmpeg determines the matching tracks when the command runs. For a filter input, select a required index such as FFix.audio(source, 0) instead. See FFix.Command.Input for examples and FFix.Command.Mapping for applying encoding settings to multiple selected tracks.