[FFmpeg-devel] Field order during libavformat write_header

[Marton Balint]

On Wed, 21 Mar 2018, Devin Heitmueller wrote:

> Hi James,
>
>> No, this is done after that, while processing a packet.
>> It is also making changes to the output codecpar after write_header()
>> was called, which is wrong.
>> 
>> libavformat used to delay writing the header until you feed it the first
>> packet, an internal functionality this code here abused.
>
> As you’re using the term “abused”, I assume you’re suggesting that it was intentionally changed to no longer wait until the first packet was received, as opposed to this being a bug in libavformat.
>
> If that’s the case, then indeed we would have to delay enabling the output until the first packet and try to figure out how badly that screws up the decklink’s pre-roll functionality for both audio and video.

I still think ffmpeg.c should be fixed instead. There is even some 
logic in ffmpeg.c to wait for a frame from all streams before 
writing the header, so it seems doable.

Also as far as I see you cannot detect the field order in the muxer if 
it is using a real codec (like v210) instead of a wrapped avframe.

Regards,
Marton