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

[Timo Rothenpieler]

On 16/05/2025 16:24, Zhao Zhili wrote:
From: Zhao Zhili <zhiliz...@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.

---
  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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