Piping MP4 from FFmpeg
Imagine you have a video file that you want to convert to MP4 using FFmpeg, and you don’t want to save the output to the disk, rather you want to pipe it to another program directly. Maybe the file is large and you want to upload it somewhere else without having to wait for the entire conversion to finish nor to store it on the disk.
Something simple like the command line below[1] unfortunately doesn’t work.
$ ffmpeg -f dhav -i 'http://192.168.2.100/somefile.dav' -c copy -f mp4 -
[mp4] muxer does not support non-seekable outputTo understand where the problem is coming from, let’s look at the structure of an MP4 file. It is comprised of data chunks called “atoms”. There are various types of atoms; we’ll make do with knowing just two of them:
The
mdattype, which contains media tracks (such as audio or video),and the
moovtype, which holds additional metadata (such as information about movie duration, codecs, etc.).
Also important to know, each atom needs to specify its size in its header. In mdat’s case, however, the size is unknown until after it’s written out entirely. So FFmpeg starts writing the mdat atom with size set to zero, then retroactively fills it in, and finally outputs the moov atom.
However you can’t seek backwards on a pipe — that’s where the complaint about non-seekable output is coming from.
An alternative to the standard MP4 structure is the “fragmented MP4”. Its moov atom is located at the beginning of the file and can be missing some fields (for example the movie duration). The tracks are split into fragments, each one producing a pair of moof and mdat atoms (the moof atom contains metadata about the mdat fragment). These fragments are small enough for FFmpeg to keep one in memory, wait for it to be complete, pipe out its moof and mdat atoms and repeat for the next one.
With a fragmented MP4 there’s no need for seeking in the output stream, which means a pipe can be used as well. To create such a file, FFmpeg must receive one additional option:
$ ffmpeg -f dhav -i 'http://192.168.2.100/somefile.dav' -c copy \
$ -movflags 'frag_keyframe+empty_moov' -f mp4 -The movflags specify that you want to create a fragmented MP4, where each keyframe starts a new fragment, the moov atom is zeroed out and is followed by the fragmented moofs and mdats.
Fragmented MP4s are less compatible. For example, the built-in video player in Windows 10 gets confused by the empty moov atom, reports zero duration and the seek bar is unusable, although it does play the file.