Improve documentation of "subfile" protocol

[sha8]

Summary of the bug:
mplayer has a -sb option. From the manual:

Seek to byte position. Useful for playback from CD-ROM images or VOB files with junk at the beginning.

While specifying an offset in dd and piping it from ffmpeg (or ffplay or ffprobe) is possible, it is inconvenient. Besides, pipe inputs are not seekable in ffplay because they are like live streams.

With an -sb option, ffmpeg (and ffplay and ffprobe) would treat the file as if it started at this byte.

Of course, -sb should also accept size suffixes. For example, 1KB = 1000 bytes. 1K = 1024 bytes. 1M = 1048576 bytes

How to reproduce:

ffplay example.img -sb 196608

ffplay -i example.img -sb 196608

ffmpeg -i example.img -sb 196608 -c copy recording.mpg 

[sha8]

Sorry for the accidental spam report. It was a misclick.

[Gyan]

You can already do this with the subfile protocol.

ffplay -start 196608 subfile:example.mp3

Of course, if you end up skipping over essential metadata, then ffmpeg may fail to open the file. Same if you do this with files where media data location is stored as global offsets.

[sha8]

Good to know! But it's currently undocumented. Please add it to the manual.

[Gyan]

It's documented at https://ffmpeg.org/ffmpeg-protocols.html#subfile

Although an example could be added to show the regular option syntax.

[MasterQuestionable]

͏    This very example seems undocumented.

͏    ​https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/subfile.c
͏    ​https://github.com/FFmpeg/FFmpeg/commit/252128561e891da8a4a3c4ea3ffa7071c443e343 ("subfile:")
͏    ​https://github.com/FFmpeg/FFmpeg/blob/790f793844390ece526ff654dc1bdddff5f5b4e8/libavformat/avio.c#L315C29-L315C37 ("subfile,")