[FFmpeg-devel] [PATCH] movenc: Add an option for hiding fragments at the end

[Dennis Sädtler]

On 2024-05-31 10:53, Martin Storsjö wrote:
> This allows ending up with a normal, non-fragmented file when
> the file is finished, while keeping the file readable if writing
> is aborted abruptly at any point. (Normally when writing a
> mov/mp4 file, the unfinished file is completely useless unless it
> is finished properly.)
>
> This results in a file where the mdat atom contains (and hides)
> all the moof atoms that were part of the fragmented file structure
> initially.
> ---
> This is, incidentally, how Apple devices do (or at least did, at some
> point) their writing of files when recording, at least with some
> of their userspace APIs.
> ---
Should the ftyp atom also be updated to remove brands no longer required 
for non-fragmented files?
I'm not sure how important that is in real-world scenarios, so it might 
not be worth it to deal with some of the additional changes required 
e.g. to deal with the new ftyp possibly being a different size.

Since coincidentally I've implemented the exact same feature in a 
different application a couple weeks ago I'll also throw in the fun fact 
that files produced this way can be smaller than regular MP4s for long 
and/or large files.
This is due to the lack of interleaving of A/V samples resulting in the 
file having much fewer but larger chunks, which means the moov atom - 
mainly the stco/co64 and stsc boxes - can be much smaller.

Also good to know that Apple thought of this as well. I had no idea 
about that, but that further justifies adopting this method for 
achieving resilient but compatible recordings in my mind.

~ Dennis