format

The demuxer to use, by FFmpeg name, as the command line's -f: the open does not probe. This is what opens headerless input, such as s16le or rawvideo, whose parameters then go in options (sample_rate, video_size and the like). A name this build does not carry fails with FFmpegError.DemuxerNotFound. Unlike formatWhitelist, which only refuses a probed format, this chooses one.