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• ffmpeg - How to (programmatically) copy a stream's side data ?

  25 mars 2021, par waldenCalms

  My app is a tag editor (eg ID3) written in Swift for macOS. I'm using ffmpeg as my library to perform the tag editing. When the user is done editing, the app should save the changes to (essentially) a modified copy of the original file. I'm able to copy everything I need (from input -> output file) EXCEPT "side data".

  


  In other words, I'm doing the programmatic equivalent of :

  


  ffmpeg -i input.mp3 -metadata key1=value1 -metadata key2=value2 -codec copy output.mp3


  


  So, output.mp3 must be identical to input.mp3, except for the few metadata changes made by the user.

  


  What I have working so far

  


  I'm able to copy the audio stream and all associated metadata (tags), as is, from input.mp3 to output.mp3.

  


  The problem

  


  My input.mp3 happens to have some side data (see below), which does not get copied over to output.mp3. How to copy side data ? To be honest, I don't even know what side data is, and the documentation is not very helpful.

  


  ffprobe gives the following output (snippet) for the input.mp3's side data that looks like :

  


  Side data:
      replaygain: track gain - -9.200000, track peak - unknown, album gain - unknown, album peak - unknown,


  


  I want THIS ^ to get copied over to output.mp3, but it does not :(

  


  Relevant code snippet (format context initialization code omitted for conciseness)

  


  let inAudioStream = inFormatContext!.pointee.streams.advanced(by: Int(audio_stream_index)).pointee!.pointee&#xA;var outAudioStream = outFormatContext!.pointee.streams.advanced(by: Int(audio_stream_index)).pointee!.pointee&#xA;            &#xA;/* Free existing side data*/&#xA;            &#xA;for i in 0..* Copy side data if present */&#xA;            &#xA;if let srcSideData = inAudioStream.side_data {&#xA;                &#xA;    let rawPtr: UnsafeMutableRawPointer? = av_mallocz_array(Int(inAudioStream.nb_side_data),&#xA;                                                            MemoryLayout<avpacketsidedata>.size)&#xA;                &#xA;    outAudioStream.side_data = UnsafeMutablePointer<avpacketsidedata>(OpaquePointer(rawPtr))&#xA;    outAudioStream.nb_side_data = inAudioStream.nb_side_data&#xA;                &#xA;    for i in 0..(OpaquePointer(data))&#xA;    }&#xA;}&#xA;            &#xA;status = avformat_write_header(outFormatContext, nil)&#xA;</avpacketsidedata></avpacketsidedata>

  


  I got this logic from this page. Is this ^ the right way to copy a stream's side data to the corresponding stream in the output file ? If not, how to do it ?

  


  I have also tried the following functions with no luck :

  


  av_stream_new_side_data(...)
av_stream_add_side_data(...)


  


  Please ask for details if required, and I will provide them. Thanks very much !

  


• ffmpeg - pts drift after seek with codec copy

  24 mars 2021, par Andrey Rikunov

  There are two ffmpeg commands. First one is used to seek and copy video chunk. Second one is used to transcode video chunk applying select filter for exact frames match.

  


  Here is how :

  


  

  1. ffmpeg -ss  -to  -copyts -i <input /> -map 0:v:0 -c copy chunk.mp4

  


  2. ffmpeg -copyts -i chunk.mp4 -vf 'select=between(pts\,\,)' transcoded_cunk.mp4

  


  


  It works fine most of the times. But for some inputs there is a little pts drift in downloaded chunk so missing frames is possible. In other words pts of the same packets (compared by hash) are shifted by several points (in my case 0,0002 sec) between input and chunked output.

  


  What is the possible reason for such pts drift ?

  


  UPDATE 1 : That's because ffmpeg set timescale=1000 in mvhd atom so edit list media time to start from looses precision. Is it possible to force mvhd timescale ?

  


  UPDATE 2 : It's not possible to change mvhd timescale because ffmpeg uses constant (MOV_TIMESCALE 1000) :

  


  https://github.com/FFmpeg/FFmpeg/blob/82bd02a2c73bb5e6b7cf5e5eba486e279f1a7358/libavformat/movenc.c#L3498

  


  UPDATE 3 : same issue discussed earlier

  


• ffmpeg audio/video sync problems when uploading to WhatsApp

  11 janvier 2021, par Luke Collins

  (Note : I have looked at related questions such as this one, but the solutions there didn't work for me since presumably their output files wouldn't play correctly in mpv, whereas mine do.)

  


  I want to send a scene from a film to my friend on WhatsApp. I run

  


  


  ffmpeg -i film.mp4 -ss 00:25:30 -t 00:03:00 -c: copy out.mp4

  


  


  to extract the clip, which produces the file out.mp4 which plays fine when I play it with mpv.

  


  If instead, I play the file in my browser (firefox), the first frame is static for a couple of seconds while audio plays, until the video starts and the video and audio are both in sync. (In other words, it seems the file has additional unnecessary audio information appended to the beginning of the file, which starts before my desired clip when played in firefox).

  


  Diagrammatically, I suspect the video contains the following information :

  


  

  


  and firefox plays it in this way :

  


  

  


  What's worse is that when I upload it to WhatsApp, it misaligns the audio with the video, instead playing it this way :

  


  

  


  How can I fix this so that the video has no additional video/audio information ?

  


  I appreciate any help with this.