[FFmpeg-devel] [PATCH 07/10] lavf: document that AVStream::codecpar may be modified by lavf after avformat_write_header(). This is assumed not to break API because it's already true (see e.g. matroskaenc handling of new AAC extradata)

[Hendrik Leppkes]

On Fri, Mar 16, 2018 at 12:29 AM, Michael Niedermayer
<michael at niedermayer.cc> wrote:
> On Wed, Mar 14, 2018 at 01:24:42AM -0500, Rodger Combs wrote:
>> ---
>>  libavformat/avformat.h | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/libavformat/avformat.h b/libavformat/avformat.h
>> index 9e87d6cdac..5f0ebfc114 100644
>> --- a/libavformat/avformat.h
>> +++ b/libavformat/avformat.h
>> @@ -1006,7 +1006,8 @@ typedef struct AVStream {
>>       *
>>       * - demuxing: filled by libavformat on stream creation or in
>>       *             avformat_find_stream_info()
>> -     * - muxing: filled by the caller before avformat_write_header()
>> +     * - muxing: filled by the caller before avformat_write_header();
>> +     * -         may be modified by libavformat afterwards
>>       */
>
> a generic "anything can change" is not really practical
> a user app would have to check every field of codecpar and possibly insert
> video scalers if resoluton changed, or all kinds of other filters and even
> different encoders if codec_id was adjusted ...
>

I generally don't like this either. codecpar describes the codec as
given to the muxer by the user. The muxer has no business changing
things around in it.
If it can derive better values somehow, let it store it internally somewhere.

- Hendrik