[FFmpeg-devel] [PATCH] avformat/apngenc: use the stream parameters extradata if no updated one is made available

[Hendrik Leppkes]

On Thu, Nov 3, 2016 at 7:39 PM, Andreas Cadhalpun
<andreas.cadhalpun at googlemail.com> wrote:
> On 03.11.2016 09:52, Hendrik Leppkes wrote:
>> Hence my point about trying to stick to a common behavior scheme
>> without looking at what *OUR* decoder needs - there could be others,
>> after all.
>> Its common to write extradata into codecpar if its present. You can
>> try to dispute that as long as you wish, but that doesn't make it any
>> less true.
>>
>> avcodec is not the only thing people use avformat with, and
>> vice-versa, so sticking to common patterns makes the life easier for
>> those.
>>
>> Do we really need to write this down somewhere to convince you, what
>> do we have the ML for then if we can't inform contributors about
>> common practice?
>
> What you need to convince me are good arguments.
> The reason why I asked about documentation is that if it's not documented,
> your argument about other decoders, that you repeated above, is void,
> because others shouldn't rely on undocumented behavior, as our
> documentation states clearly [1]:
> "Behavior in undocumented situations may change slightly (and be documented)."
>
> So if you want to be able to rely on the behavior you describe, you'll
> have to get it documented.
> As you seem to care about this a lot, I don't understand why you appear
> to be opposed to the idea of documenting it.
>
>

Since you want docs, I even found one extremely specific to this
particular case:
https://ffmpeg.org/ffmpeg-formats.html#apng

"All headers, but the PNG signature, up to (but not including) the
first fcTL chunk are transmitted as extradata."

I'm sure you will argue that side-data is still extradata of some
sort, but fact is the apng demuxer changed behavior recently, and the
documentation suggests a certain content of extradata is to be
expected.

- Hendrik