How can I determine if a codec / container combination is compatible with FFmpeg?

问题

I'm looking at re-muxing some containers holding audio and video such that I extract the best, first audio stream, and store it in a new container where e.g. only the audio stream is present.

The output context for FFmpeg is created like so:

AVFormatContext* output_context = NULL;
avformat_alloc_output_context2( &output_context, NULL, "mp4", NULL );

I have a shortlist of acceptable outputs, e.g. MP4, M4A, etc … essentially those that are readable by Apple's Audio File Services:

kAudioFileAIFFType              = 'AIFF',
kAudioFileAIFCType              = 'AIFC',
kAudioFileWAVEType              = 'WAVE',
kAudioFileSoundDesigner2Type    = 'Sd2f',
kAudioFileNextType              = 'NeXT',
kAudioFileMP3Type               = 'MPG3',   // mpeg layer 3
kAudioFileMP2Type               = 'MPG2',   // mpeg layer 2
kAudioFileMP1Type               = 'MPG1',   // mpeg layer 1
kAudioFileAC3Type               = 'ac-3',
kAudioFileAAC_ADTSType          = 'adts',
kAudioFileMPEG4Type             = 'mp4f',
kAudioFileM4AType               = 'm4af',
kAudioFileM4BType               = 'm4bf',
kAudioFileCAFType               = 'caff',
kAudioFile3GPType               = '3gpp',
kAudioFile3GP2Type              = '3gp2',
kAudioFileAMRType               = 'amrf'

My question is this : is there an easy API in FFmpeg that can be leveraged to choose a compatible output container given the codec the audio stream is in?

回答1:

For each individual muxer, there is usually a codec tag writing function.That function will check against a list in another source file or work through a switch statement in the same. There is no central roster or container-matching utility function. Your best bet is to identify the codec id in libavcodec/allcodecs.c and then grep in libavformat/ for that ID, particularly within files suffixed with enc e.g. matroskaenc.c.

回答2:

There's a dynamic approach to that problem. This enumerates the codecs for each container, but you also get the inverse:

// enumerate all codecs and put into list
std::vector<AVCodec*> encoderList;
AVCodec * codec = nullptr;
while (codec = av_codec_next(codec))
{
    // try to get an encoder from the system
    auto encoder = avcodec_find_encoder(codec->id);
    if (encoder)
    {
        encoderList.push_back(encoder);
    }
}
// enumerate all containers
AVOutputFormat * outputFormat = nullptr;
while (outputFormat = av_oformat_next(outputFormat))
{
    for (auto codec : encoderList)
    {
        // only add the codec if it can be used with this container
        if (avformat_query_codec(outputFormat, codec->id, FF_COMPLIANCE_STRICT) == 1)
        {
            // add codec for container
        }
    }
}

If you just want specific containers or codecs you can use a whitelist with their name or id fields and use that when enumerating.

来源：https://stackoverflow.com/questions/47276731/how-can-i-determine-if-a-codec-container-combination-is-compatible-with-ffmpeg

标签

audio

ffmpeg

libavcodec

libav

libavformat