How to output fragmented mp4 with ffmpeg?

Question

ffmpeg -i infile.avi out.mp4 outputs non-fragmented MP4.

How do I obtain fragmented mp4?

Update A fragmented mp4 file is internally divided into several back-to-back chunks or MPEG-4 movie fragments. Each chunk has its own moof atom - so there are several moof atoms interleaved in the file instead of a single moov at the end as in the case of an unfragmented mp4. This makes it easier to stream over slow networks where buffering is involved

There are several tools like mp4box that convert a normal mp4 to a fragmented one. Unfortunately we cannot use something like this

ffmpeg <options to output mp4> | mp4box

since ffmpeg does not produce seekable output while producing mp4 containers.

Answer 1

This should do the trick:

ffmpeg -re -i infile.ext -g 52 \
-strict experimental -acodec aac -ab 64k -vcodec libx264 -vb 448k \
-f mp4 -movflags frag_keyframe+empty_moov \
output.mp4

• frag_keyframe causes fragmented output,
• empty_moov will cause output to be 100% fragmented; without this the first fragment will be muxed as a short movie (using moov) followed by the rest of the media in fragments,
• -re is useful when live streaming (use input media frame rate), do not use it if you are creating a file,
• -g 52 forces (at least) every 52nd frame to be a keyframe

To calculate a healthy keyframe interval please see the paragraphs about fragment sizes in the docs of my streaming server. - You can also consider using WebM which is a free alternative to H.264 (and has better support on some platforms than fragmented mp4).

Important note: FFMpeg's muxer will set the Duration in both tkhd and mdhd atoms to 0xffffffff for each track. This causes problems in some players (for example Quicktime will not play such files). You should find a tool and change it to zero (0x00000000).