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• Losing quality when encoding with ffmpeg

  22 mai 2016, par lupod

  I am using the c libraries of ffmpeg to read frames from a video and create an output file that is supposed to be identical to the input.
  However, somewhere during this process some quality gets lost and the result is "less sharp". My guess is that the problem is the encoding and that the frames are too compressed (also because the size of the file decreases quite significantly). Is there some parameter in the encoder that allows me to control the quality of the result ? I found that AVCodecContext has a compression_level member, but changing it that does not seem to have any effect.

  I post here part of my code in case it could help. I would say that something must be changed in the init function of OutputVideoBuilder when I set the codec. The AVCodecContext that is passed to the method is the same of InputVideoHandler.
  Here are the two main classes that I created to wrap the ffmpeg functionalities :

  // This class opens the video files and sets the decoder
  
  class InputVideoHandler {
  
  public:
  
    InputVideoHandler(char* name);
  
    ~InputVideoHandler();
  
    AVCodecContext* getCodecContext();
  
    bool readFrame(AVFrame* frame, int* success);
  
  
  
  private:
  
    InputVideoHandler();
  
    void init(char* name);
  
    AVFormatContext* formatCtx;
  
    AVCodec* codec;
  
    AVCodecContext* codecCtx;
  
    AVPacket packet;
  
    int streamIndex;
  
  };
  
  
  
  void InputVideoHandler::init(char* name) {
  
    streamIndex = -1;
  
    int numStreams;
  
  
  
    if (avformat_open_input(&formatCtx, name, NULL, NULL) != 0)
  
      throw std::exception("Invalid input file name.");
  
  
  
    if (avformat_find_stream_info(formatCtx, NULL)<0)
  
      throw std::exception("Could not find stream information.");
  
  
  
    numStreams = formatCtx->nb_streams;
  
  
  
    if (numStreams < 0)
  
      throw std::exception("No streams in input video file.");
  
  
  
    for (int i = 0; i < numStreams; i++) {
  
      if (formatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO) {
  
        streamIndex = i;
  
        break;
  
      }
  
    }
  
  
  
    if (streamIndex < 0)
  
      throw std::exception("No video stream in input video file.");
  
  
  
    // find decoder using id
  
    codec = avcodec_find_decoder(formatCtx->streams[streamIndex]->codec->codec_id);
  
    if (codec == nullptr)
  
      throw std::exception("Could not find suitable decoder for input file.");
  
  
  
    // copy context from input stream
  
    codecCtx = avcodec_alloc_context3(codec);
  
    if (avcodec_copy_context(codecCtx, formatCtx->streams[streamIndex]->codec) != 0)
  
      throw std::exception("Could not copy codec context from input stream.");
  
  
  
    if (avcodec_open2(codecCtx, codec, NULL) < 0)
  
      throw std::exception("Could not open decoder.");
  
  }
  
  
  
  // frame must be initialized with av_frame_alloc() before!
  
  // Returns true if there are other frames, false if not.
  
  // success == 1 if frame is valid, 0 if not.
  
  bool InputVideoHandler::readFrame(AVFrame* frame, int* success) {
  
    *success = 0;
  
    if (av_read_frame(formatCtx, &packet) < 0)
  
      return false;
  
    if (packet.stream_index == streamIndex) {
  
      avcodec_decode_video2(codecCtx, frame, success, &packet);
  
    }
  
    av_free_packet(&packet);
  
    return true;
  
  }
  
  
  
  // This class opens the output and write frames to it
  
  class OutputVideoBuilder{
  
  public:
  
    OutputVideoBuilder(char* name, AVCodecContext* inputCtx);
  
    ~OutputVideoBuilder();
  
    void writeFrame(AVFrame* frame);
  
    void writeVideo();
  
  
  
  private:
  
    OutputVideoBuilder();
  
    void init(char* name, AVCodecContext* inputCtx);
  
    void logMsg(AVPacket* packet, AVRational* tb);
  
    AVFormatContext* formatCtx;
  
    AVCodec* codec;
  
    AVCodecContext* codecCtx;
  
    AVStream* stream;
  
  };
  
  
  
  void OutputVideoBuilder::init(char* name, AVCodecContext* inputCtx) {
  
    if (avformat_alloc_output_context2(&formatCtx, NULL, NULL, name) < 0)
  
      throw std::exception("Could not determine file extension from provided name.");
  
  
  
    codec = avcodec_find_encoder(inputCtx->codec_id);
  
    if (codec == nullptr) {
  
      throw std::exception("Could not find suitable encoder.");
  
    }
  
  
  
    codecCtx = avcodec_alloc_context3(codec);
  
    if (avcodec_copy_context(codecCtx, inputCtx) < 0)
  
      throw std::exception("Could not copy output codec context from input");
  
  
  
    codecCtx->time_base = inputCtx->time_base;
  
    codecCtx->compression_level = 0;
  
  
  
    if (avcodec_open2(codecCtx, codec, NULL) < 0)
  
      throw std::exception("Could not open encoder.");
  
  
  
    stream = avformat_new_stream(formatCtx, codec);
  
    if (stream == nullptr) {
  
      throw std::exception("Could not allocate stream.");
  
    }
  
  
  
    stream->id = formatCtx->nb_streams - 1;
  
    stream->codec = codecCtx;
  
    stream->time_base = codecCtx->time_base;
  
  
  
  
  
  
  
    av_dump_format(formatCtx, 0, name, 1);
  
    if (!(formatCtx->oformat->flags & AVFMT_NOFILE)) {
  
      if (avio_open(&formatCtx->pb, name, AVIO_FLAG_WRITE) < 0) {
  
        throw std::exception("Could not open output file.");
  
      }
  
    }
  
  
  
    if (avformat_write_header(formatCtx, NULL) < 0) {
  
      throw std::exception("Error occurred when opening output file.");
  
    }
  
  
  
  }
  
  
  
  void OutputVideoBuilder::writeFrame(AVFrame* frame) {
  
    AVPacket packet = { 0 };
  
    int success;
  
    av_init_packet(&packet);
  
  
  
    if (avcodec_encode_video2(codecCtx, &packet, frame, &success))
  
      throw std::exception("Error encoding frames");
  
  
  
    if (success) {
  
      av_packet_rescale_ts(&packet, codecCtx->time_base, stream->time_base);
  
      packet.stream_index = stream->index;
  
      logMsg(&packet,&stream->time_base);
  
      av_interleaved_write_frame(formatCtx, &packet);
  
    }
  
    av_free_packet(&packet);
  
  }

  This is the part of the main function that reads and write frames :

   while (inputHandler->readFrame(frame,&gotFrame)) {
  
  
  
      if (gotFrame) {
  
        try {
  
          outputBuilder->writeFrame(frame);
  
        }
  
        catch (std::exception e) {
  
          std::cout << e.what() << std::endl;
  
          return -1;
  
        }
  
      }
  
    }

• lavc/videotoolboxenc : set extradata when opening codec

  29 mai 2016, par Rick Kern

  lavc/videotoolboxenc : set extradata when opening codec
  VideoToolbox doesn’t supply parameter sets until the first frame is done
  
  encoding. This spins up a temporary encoder and encodes a single frame to
  
  get this data.
  Signed-off-by : Rick Kern <kernrj@gmail.com>
  

  • [DH] libavcodec/videotoolboxenc.c

• avformat/allformats : Making av_register_all() thread-safe.

  7 avril 2016, par Vivekanand

  avformat/allformats : Making av_register_all() thread-safe.
  When multiple threads tries to call av_register_all(), the first thread sets
  
  initialized to 1 and do the register process. At the same time, other thread might
  
  also call av_register_all(), which returns immediately because initialized is set to 1
  
  (even when it has not completed registering codecs). We can avoid this problem
  
  if we set initialised to 1 while exiting from function.
  Github : Closes #196
  

  • [DH] libavformat/allformats.c