[Libav-user] Change the default audio and video codec loaded by avformat_alloc_output_context2

[Guangyu Sun]

Hi Mythili,

Check these two functions. Setting flvflags to the options should work.

int avformat_write_header(AVFormatContext *s, AVDictionary **options);
int avformat_init_output(AVFormatContext *s, AVDictionary **options);

Thanks,
Guangyu

On Tue, Jun 8, 2021 at 9:46 PM Mythili Vishalini Anbazhagan <
manbazhagan27 at gmail.com> wrote:

> Hello,
> Thank you so much for the information. I will try that and see.
>
> I have found a workaround. Don't know if this is the correct way of doing
> this though.
> Created AVOutputFormat using  "AVOutputFormat* oformat = av_guess_format(
> "flv", NULL, NULL)" and then manually edited the audio and video codec
> fields in "oformat" before giving it to avformat_alloc_output_context2.
> It works and I'm able to stream audio and video.
>
> But I get the following warning:
> "Failed to update header with correct duration.Failed to update header
> with correct filesize."
> In the command line, I rectified this error with the flag -flvflags
> no_duration_filesize.
> Could you tell me how to set this flag in the API ?
>
> Thank you so much.
>
> Best Regards,
> Mythili Vishalini.
>
>
> On Tue, Jun 8, 2021 at 11:19 PM Guangyu Sun via Libav-user <
> libav-user at ffmpeg.org> wrote:
>
>> This might be what you are looking for:
>> AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c);
>>
>> Specify the correct codec when adding streams to the AVFormatContext
>> created by avformat_alloc_output_context2().
>>
>> On Mon, Jun 7, 2021 at 11:39 PM Mythili Vishalini Anbazhagan <
>> manbazhagan27 at gmail.com> wrote:
>>
>>> Hi,
>>> I'm using ffmpeg libav library for live streaming via RTMP. I want to
>>> know how to give my choice of audio and video codec for the particular
>>> format in avformat_alloc_output_context2.
>>>
>>> In Detail:
>>> The following command works perfectly for me.
>>> ffmpeg -re -stream_loop -1 -i ~/Downloads/Microsoft_Surface.mp4 -vcodec
>>> copy -c:a aac -b:a 160k -ar 44100 -strict -2 -f flv -flvflags
>>> no_duration_filesize rtmp://192.168.1.7/live/surface
>>> In the output, I have set my audio codec to be aac and copied the video
>>> codec from input, which is H264.
>>> I want to emulate this in the library, but don't know how to.
>>> avformat_alloc_output_context2, with parameter format name as "flv",
>>> sets oformat audio codec to ADPCM_SWF and video codec to FLV1. How to
>>> change that to AAC and H264 ?
>>>
>>> So far, used av_guess_format to construct AVOutputFormat. It accepts
>>> only format as input. And I don't know where to mention audio and video
>>> codec.
>>> Also tried giving filename to avformat_alloc_output_context2 with the
>>> rest of the parameters NULL. The file has AAC audio and H264 video. But
>>> still ffmpeg loads oformat with ADPCM_SWF audio and FLV1 video codecs.
>>>
>>> Any hint/guidance is hugely appreciated. Thank you.
>>>
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