Re: [FFmpeg-devel] [PATCH] avcodec/avformat: Added codec_name to AVCodecContext and AVCodecParameters

[Bernardo Pilarz via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

On 03/07/2024 11:10, Anton Khirnov wrote:
> Quoting Bernardo Pilarz via ffmpeg-devel (2024-07-03 10:10:15)
>>>> +    char *codec_name;
>>>>         const struct AVCodec  *codec;
>>>>         enum AVCodecID     codec_id; /* see AV_CODEC_ID_xxx */
>>>>
>>> Adding a new field here is an ABI break, it would need to go at the
>>> end of the struct.
>>>
>>> In general, I feel like this might be better served to go into
>>> metadata though, especially as very few containers have a string codec
>>> identifier to begin with.
>> I would be very glad to do it the right way, but I need some guidance
>> since this is the first time I try to contribute to FFmpeg.
>>
>> The problem that I am trying to solve is receiving metadata from an RTSP
>> stream (in example, ONVIF metadata identified by the codec name
>> 'vcd.onvif.metadata').
>> This is data that the application will want to handle on its own (and
>> not through FFmpeg).
>>
>> Can you guide me on how to do this properly?
> First of all, this should definitely NOT go into AVCodecContext, since
> the entire premise is that we have no codec ID and hence no decoders or
> encoders, so a codec context makes no sense.
>
> Next, the idea that you can define a generic container-independent
> "codec name" sounds dubious to me, as they are not universally
> standardized. You'd either have to define precisely what can the API
> caller expect in there, or make this a private AV_OPT_FLAG_EXPORT option
> of your demuxer. I'd suggest the second, as it's much simpler, though
> IIRC we do not yet support per-stream private options.
>
I ended up adding it to the AVCodecContext because 
'avformat_find_stream_info()' invokes 'avcodec_parameters_to_context()' 
followed by 'avcodec_parameters_from_context()', thereby overwriting the 
entire 'codecpar' field of each stream in my format context.
This also means that an AVCodecContext is in fact created even for 
streams with no codec ID.

It's not clear to me how I can use AV_OPT_FLAG_EXPORT (it's my fault 
because I don't know the library so well).
It might very well be the solution I am looking for, but I need some 
help in understanding how to implement it.

The basic problem is I need to get the "codec name" information that is 
available in 'sdp_parse_rtpmap()' and is completely lost when the codec 
is not supported by FFmpeg.
By the way, we are talking of AVMEDIA_TYPE_DATA streams, and codecs that 
are potentially custom/not widespread and therefore should probably be 
handled by the application rather than by ffmpeg itself.

P.S.: I am currently only using FFmpeg to grab an RTSP stream. The audio 
and video streams might very well get decoded using FFmpeg later on, but 
so far I am only concerned with getting the compressed packets and being 
able to read their declared codec names.


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