[FFmpeg-devel] [PATCH] avformat/flvenc: Specify codec tag with MKTAG

[Zhao Zhili]

> On May 16, 2025, at 22:52, Timo Rothenpieler <timo at rothenpieler.org> wrote:
> 
> On 16/05/2025 16:24, Zhao Zhili wrote:
>> From: Zhao Zhili <zhilizhao at tencent.com <mailto:zhilizhao at tencent.com>>
>> ffmpeg -i input.mp4 -c copy -tag:v av01 output.flv
>> [flv @ 0x143204080] Tag av01 incompatible with output codec id '225' (10va)
> 
> I don't quite understand what causes this.
> Is this an issue when running on big endian architectures?
> I'm pretty sure I tested all combinations of codecs with muxing and demuxing, and never ran into that error.

The key point is when specify tag via command line, e.g., -tag:v av01, it’s
passed to AVCodecParameters codec_tag in little endian.

You didn’t see the error because without specify the tag explicitly, codec_tag is copied
from AVOutputFormat codec_tag to AVCodecParameters codec_tag, so they are
the same.

Another example is codec_mp4_tags.

> 
>> ---
>>  libavformat/flvenc.c | 26 +++++++++++++++++---------
>>  1 file changed, 17 insertions(+), 9 deletions(-)
>> diff --git a/libavformat/flvenc.c b/libavformat/flvenc.c
>> index f026f0e53b..e92828b197 100644
>> --- a/libavformat/flvenc.c
>> +++ b/libavformat/flvenc.c
>> @@ -52,9 +52,9 @@ static const AVCodecTag flv_video_codec_ids[] = {
>>      { AV_CODEC_ID_VP6,      FLV_CODECID_VP6 },
>>      { AV_CODEC_ID_VP6A,     FLV_CODECID_VP6A },
>>      { AV_CODEC_ID_H264,     FLV_CODECID_H264 },
>> -    { AV_CODEC_ID_HEVC,     MKBETAG('h', 'v', 'c', '1') },
>> -    { AV_CODEC_ID_AV1,      MKBETAG('a', 'v', '0', '1') },
>> -    { AV_CODEC_ID_VP9,      MKBETAG('v', 'p', '0', '9') },
>> +    { AV_CODEC_ID_HEVC,     MKTAG('h', 'v', 'c', '1') },
>> +    { AV_CODEC_ID_AV1,      MKTAG('a', 'v', '0', '1') },
>> +    { AV_CODEC_ID_VP9,      MKTAG('v', 'p', '0', '9') },
>>      { AV_CODEC_ID_NONE,     0 }
>>  };
>>  @@ -69,10 +69,10 @@ static const AVCodecTag flv_audio_codec_ids[] = {
>>      { AV_CODEC_ID_PCM_MULAW,  FLV_CODECID_PCM_MULAW  >> FLV_AUDIO_CODECID_OFFSET },
>>      { AV_CODEC_ID_PCM_ALAW,   FLV_CODECID_PCM_ALAW   >> FLV_AUDIO_CODECID_OFFSET },
>>      { AV_CODEC_ID_SPEEX,      FLV_CODECID_SPEEX      >> FLV_AUDIO_CODECID_OFFSET },
>> -    { AV_CODEC_ID_OPUS,       MKBETAG('O', 'p', 'u', 's') },
>> -    { AV_CODEC_ID_FLAC,       MKBETAG('f', 'L', 'a', 'C') },
>> -    { AV_CODEC_ID_AC3,        MKBETAG('a', 'c', '-', '3') },
>> -    { AV_CODEC_ID_EAC3,       MKBETAG('e', 'c', '-', '3') },
>> +    { AV_CODEC_ID_OPUS,       MKTAG('O', 'p', 'u', 's') },
>> +    { AV_CODEC_ID_FLAC,       MKTAG('f', 'L', 'a', 'C') },
>> +    { AV_CODEC_ID_AC3,        MKTAG('a', 'c', '-', '3') },
>> +    { AV_CODEC_ID_EAC3,       MKTAG('e', 'c', '-', '3') },
>>      { AV_CODEC_ID_NONE,       0 }
>>  };
>>  @@ -348,7 +348,11 @@ static void write_metadata(AVFormatContext *s, unsigned int ts)
>>          }
>>            put_amf_string(pb, "videocodecid");
>> -        put_amf_double(pb, flv->video_par->codec_tag);
>> +        uint32_t tag = flv->video_par->codec_tag;
>> +        // check and convert to fourcc
>> +        if (tag & 0xFFFFFF00)
>> +            tag = av_bswap32(tag);
>> +        put_amf_double(pb, tag);
> 
> Shouldn't this be completely equivalent to having MKBETAG generate it in big-endian order from the get-go?
> 
>>      }
>>        if (flv->audio_par) {
>> @@ -365,7 +369,11 @@ static void write_metadata(AVFormatContext *s, unsigned int ts)
>>          put_amf_bool(pb, flv->audio_par->ch_layout.nb_channels == 2);
>>            put_amf_string(pb, "audiocodecid");
>> -        put_amf_double(pb, flv->audio_par->codec_tag);
>> +        uint32_t tag = flv->audio_par->codec_tag;
>> +        // check and convert to fourcc
>> +        if (tag & 0xFFFFFF00)
>> +            tag = av_bswap32(tag);
>> +        put_amf_double(pb, tag);
>>      }
>>        if (flv->data_par) {
> 
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