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• libav : Store h264 frames in mp4 container

  25 janvier 2024, par ImJustACowLol

  I'm making a C++ application that retrieves frames from a camera and then encodes each frame with a H264 encoder (not using libav). This encoded H264 frame is then kept in memory as a void *mem as I need to do several things with the encoded frame.

  


  One of the things I need to do, is store the frames (so the void *mem pointers) in a .mp4 container using libavcodec/libavformat. I do NOT want to transcode each frame, I just want to store them directly into the mp4 container.

  


  Preferably for each individual frame that I push through, I get the resulting data as a return type from the function (not sure if this is possible ?). If this is not possible, then writing to a file directly is OK as well.

  


  How does one go about doing this with libav ?

  


  The only part I have got so far, and where I'm getting stuck, is this :

  


  /*
some private fields accessible in MP4Muxer:
int frameWidth_, frameHeight_, frameRate_, srcBitRate_;
*/


void MP4Muxer::muxFrame(void *mem, size_t len, int64_t timestamp, bool keyFrame) {
    const AVOutputFormat* outputFormat = av_guess_format("mp4", NULL, NULL);
    AVFormatContext* outputFormatContext = avformat_alloc_context();
    outputFormatContext->oformat = outputFormat;
    AVStream* videoStream = avformat_new_stream(outputFormatContext, NULL);

    videoStream->codecpar->codec_type = AVMEDIA_TYPE_VIDEO;
    videoStream->codecpar->codec_id = AV_CODEC_ID_H264;
    videoStream->codecpar->width = frameWidth_;
    videoStream->codecpar->height = frameHeight_;
    videoStream->avg_frame_rate = (AVRational) {frameRate_, 1};
    videoStream->time_base = (AVRational) {1, 90000};

}


  


  How do I continue from here ? Are there any good resources I can follow ? There are some resources I found online, but all of them either write the output directly to a file, read input directly from streams/files etc. so I have a hard time translating them to my needs.

  


• movenc : Add a flag for indicating a discontinuous fragment

  20 novembre 2014, par Martin Storsjö

  movenc : Add a flag for indicating a discontinuous fragment
  This allows creating a later mp4 fragment without sequentially
  
  writing the earlier ones before (when called from a segmenter).
  Normally when writing a fragmented mp4 file sequentially, the
  
  first timestamps of a fragment are adjusted to match the
  
  end of the previous fragment, to make sure the timestamp is the
  
  same, even if it is calculated as the sum of previous fragment
  
  durations. (And for the first packet in a file, the offset of
  
  the first packet is written using an edit list.)
  When writing an individual mp4 fragment discontinuously like this
  
  (with potentially writing the earlier fragments separately later),
  
  there’s a risk of getting a gap in the timeline if the duration
  
  field of the last packet in the previous fragment doesn’t match up
  
  with the start time of the next fragment.
  Using this requires setting -avoid_negative_ts make_non_negative
  
  (or -avoid_negative_ts 0).
  Signed-off-by : Martin Storsjö <martin@martin.st>
  

  • [DBH] libavformat/movenc.c
  • [DBH] libavformat/movenc.h

• movenc : Allow writing a DASH sidx atom at the start of files

  21 octobre 2014, par Martin Storsjö

  movenc : Allow writing a DASH sidx atom at the start of files
  This is mapped to the faststart flag (which in this case
  
  perhaps should be called "shift and write index at the
  
  start of the file"), which for fragmented files will
  
  write a sidx index at the start.
  When segmenting DASH into files, there’s usually one sidx
  
  at the start of each segment (although it’s not clear to me
  
  whether that actually is necessary). When storing all of it
  
  in one file, the MPD doesn’t necessarily need to describe
  
  the individual segments, but the offsets of the fragments can be
  
  fetched from one large sidx atom at the start of the file. This
  
  allows creating files for the DASH ISO BMFF on-demand profile.
  Signed-off-by : Martin Storsjö <martin@martin.st>
  

  • [DBH] libavformat/movenc.c
  • [DBH] libavformat/movenc.h