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
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