[FFmpeg-devel] [PATCH 3/3 v2] avformat/movenc: set delay_moov flag when writing DASH

[Martin Storsjö]

On Fri, 22 Nov 2024, James Almer wrote:

> On 11/22/2024 5:32 AM, Martin Storsjö wrote:
>> On Thu, 21 Nov 2024, James Almer wrote:
>> 
>> I'm a little reluctant to do this; IIRC the delay_moov flag significantly 
>> changes the sequence of what boxes gets output at what time, which affects 
>> things for API users integrating this into segmentation setups. (I presume 
>> that's why you changed the movenc test case as well?)
>
> The changes in md5 hashes there are only the missing dash brand in the 
> output, afaict. I removed the dash flag (which is not what the test was about 
> anyway) to make sure delay_moov was not included in the test that only wanted 
> empty_moov.

Ah, I see.

>> That's why I'd like to keep delay_moov an explicit opt-in - even if it 
>> kinda is necessary to get the timing entirely correct. Users that don't 
>> need it, and rely on getting the whole moov during avformat_write_header, 
>> would be broken by this change.
>
> But who uses the mov muxer with the dash movflag on its own instead of 
> through the dash muxer, which explicitly enables delay_moov? afaik that flag 
> only ensures the brand is added.
> Functionality wise it does nothing more than select other flags. Or rather, 
> it should only do that, right? And if it does not, whatever code looks for it 
> should be changed to look for the fragment flag instead.

I would expect that many users use it, as an umbrella option for "modern 
fragmented mp4 with all the flavours you need for segmented use". A more 
modern similar umbrella flag would probably be "cmaf" though, but that 
flag was added (much) later.

Yes, it's also possible to explicitly request 
"empty_moov+default_base_moof", but it's not possible to directly set the 
flag corresponding to FF_MOV_FLAG_FRAGMENT (which is "make sure we're 
fragmenting; if the user didn't specify an option for fragmentation, pick 
a default one"). But just picking cmaf or dash as umbrella option for 
these setups is convenient.


Any project doing segmented mp4, by segmenting externally (rather than 
using our muxers), can likely be using this flag - I use it myself in this 
fashion in $dayjob code. Yes I can obviously change that now that I'm 
aware of this change, but I would believe that there are a number of other 
users using it in a similar way - Hyrum's Law etc.

And while we do have our segmenting muxers (dashenc etc), I would expect 
that many of the advanced use cases do the segmenting themselves.

I.e., I was the one to add the dash flag back in the day, and I would be 
surprised by this change, if I hadn't caught it in review.

And if the prime use case in mind is the dash muxer, I don't see why the 
dash muxer can't keep on passing the delay_moov flag separately?

// Martin