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• Creating a usable H.264 video file

  4 mai 2019, par Ethan McTague

  I am trying to use libavcodec to generate an mp4 video file from individual frames. Each input frame is a qt QImage, and the output file is written to using the Qt QFile class.

  I’ve done this through a VideoTarget class which opens the given ’target’ file when initialized, records frames when addFrame(image) is called, and then saves/closes the file when its destructor is called.

  The class has the following fields :

  AVCodec* m_codec = nullptr;
  
  AVCodecContext *m_context = nullptr;
  
  AVPacket* m_packet = nullptr;
  
  AVFrame* m_frame = nullptr;
  
  
  
  QFile m_target;

  And looks like this :

  VideoTarget::VideoTarget(QString target, QObject *parent) : QObject(parent), m_target(target)
  
  {
  
      // Find video codec
  
      m_codec = avcodec_find_encoder_by_name("libx264rgb");
  
      if (!m_codec) throw std::runtime_error("Unable to find codec.");
  
  
  
      // Make codec context
  
      m_context = avcodec_alloc_context3(m_codec);
  
      if (!m_context) throw std::runtime_error("Unable to allocate codec context.");
  
  
  
      // Make codec packet
  
      m_packet = av_packet_alloc();
  
      if (!m_packet) throw std::runtime_error("Unable to allocate packet.");
  
  
  
      // Configure context
  
      m_context->bit_rate = 400000;
  
      m_context->width = 1280;
  
      m_context->height = 720;
  
      m_context->time_base = (AVRational){1, 60};
  
      m_context->framerate = (AVRational){60, 1};
  
      m_context->gop_size = 10;
  
      m_context->max_b_frames = 1;
  
      m_context->pix_fmt = AV_PIX_FMT_RGB24;
  
  
  
      if (m_codec->id == AV_CODEC_ID_H264)
  
          av_opt_set(m_context->priv_data, "preset", "slow", 0);
  
  
  
      // Open Codec
  
      int ret = avcodec_open2(m_context, m_codec, nullptr);
  
      if (ret < 0) {
  
          throw std::runtime_error("Unable to open codec.");
  
      }
  
  
  
      // Open file
  
      if (!m_target.open(QIODevice::WriteOnly))
  
          throw std::runtime_error("Unable to open target file.");
  
  
  
      // Allocate frame
  
      m_frame = av_frame_alloc();
  
      if (!m_frame) throw std::runtime_error("Unable to allocate frame.");
  
  
  
      m_frame->format = m_context->pix_fmt;
  
      m_frame->width = m_context->width;
  
      m_frame->height = m_context->height;
  
      m_frame->pts = 0;
  
  
  
      ret = av_frame_get_buffer(m_frame, 24);
  
      if (ret < 0) throw std::runtime_error("Unable to allocate frame buffer.");
  
  }
  
  
  
  void VideoTarget::addFrame(QImage &image)
  
  {
  
      // Ensure frame data is writable
  
      int ret = av_frame_make_writable(m_frame);
  
      if (ret < 0) throw std::runtime_error("Unable to make frame writable.");
  
  
  
      // Prepare image
  
      for (int y = 0; y < m_context->height; y++) {
  
          for (int x = 0; x < m_context->width; x++) {
  
              auto pixel = image.pixelColor(x, y);
  
              int pos = (y * 1024 + x) * 3;
  
              m_frame->data[0][pos] = pixel.red();
  
              m_frame->data[0][pos + 1] = pixel.green();
  
              m_frame->data[0][pos + 2] = pixel.blue();
  
          }
  
      }
  
  
  
      m_frame->pts++;
  
  
  
      // Send the frame
  
      ret = avcodec_send_frame(m_context, m_frame);
  
      if (ret < 0) throw std::runtime_error("Unable to send AV frame.");
  
  
  
      while (ret >= 0) {
  
          ret = avcodec_receive_packet(m_context, m_packet);
  
          if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
  
              return;
  
          else if (ret < 0) throw std::runtime_error("Error during encoding.");
  
  
  
          m_target.write((const char*)m_packet->data, m_packet->size);
  
          av_packet_unref(m_packet);
  
      }
  
  }
  
  
  
  VideoTarget::~VideoTarget()
  
  {
  
      int ret = avcodec_send_frame(m_context, nullptr);
  
      if (ret < 0) throw std::runtime_error("Unable to send AV null frame.");
  
  
  
      while (ret >= 0) {
  
          ret = avcodec_receive_packet(m_context, m_packet);
  
          if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
  
              return;
  
          else if (ret < 0) throw std::runtime_error("Error during encoding.");
  
  
  
          m_target.write((const char*)m_packet->data, m_packet->size);
  
          av_packet_unref(m_packet);
  
      }
  
  
  
      // Magic number at the end of the file
  
      uint8_t endcode[] = { 0, 0, 1, 0xb7 };
  
      m_target.write((const char*)endcode, sizeof(endcode));
  
      m_target.close();
  
  
  
      // Free codec stuff
  
      avcodec_free_context(&m_context);
  
      av_frame_free(&m_frame);
  
      av_packet_free(&m_packet);
  
  }

  When used, the class seems to work, and data is written to the file, except I am unable to play back the resulting file in any application.

  My main suspect is these lines :

      // Prepare image
  
      for (int y = 0; y < m_context->height; y++) {
  
          for (int x = 0; x < m_context->width; x++) {
  
              auto pixel = image.pixelColor(x, y);
  
              int pos = (y * 1024 + x) * 3;
  
              m_frame->data[0][pos] = pixel.red();
  
              m_frame->data[0][pos + 1] = pixel.green();
  
              m_frame->data[0][pos + 2] = pixel.blue();
  
          }
  
      }

  The libavcodec documentation was extremely vague regarding the layout of image data, so I effectively had to guess and be happy with the first thing that didn’t crash, so chances are I’m writing this incorrectly. There’s also the issue of size mismatch between my pixel color data calls (giving int values) and the 24-bits-per-pixel RGB format I have selected.

  How do I tweak this code to output actual, functioning video files ?

• ffmpeg record before and after applying filters

  7 mai 2019, par Mark

  I’m trying to use ffmpeg to do the following things :

  • acquire input streaming [OK]
  • save to disk the video as is [TODO]
  • apply filter, i.e. drawtext [OK]
  • save to disk the overlayed video [OK]
  • preview the overlayed video [OK]

  Here my current command line :

  ffmpeg -rtsp_transport tcp -i  -vf "[in]drawtext=textfile='text.txt': reload=1: font=arial: fontcolor=red: fontsize=80: box=1: boxcolor=yellow@0.5: boxborderw=5: x=(w-text_w)/2: y=(h-text_h)/2[out]" -vcodec libx264 -f tee -map 0:v "[f=mp4]test.mp4|[f=nut]pipe:" | ffplay pipe:

  The only thing I don’t understand how to do is the recording of the video before drawtext. I guess I have to create another tee :

  INPUT ---> TEE ---->RECORD
  
                  |
  
                  |-->FILTERS---> TEE ---->RECORD
  
                                       |
  
                                       |-->PLAY

  So I tried with this cumbersome command :

  ffmpeg -rtsp_transport tcp -i  -vcodec libx264 -f tee -map 0:v "[f=mp4]before.mp4|[f=nut]pipe:" | ffmpeg -f mp4 -i pipe: -vf "[in]drawtext=textfile='text.txt': reload=1: font=arial: fontcolor=red: fontsize=80: box=1: boxcolor=yellow@0.5: boxborderw=5: x=(w-text_w)/2: y=(h-text_h)/2[out]" -vcodec libx264 -f tee -map 0:v "[f=mp4]after.mp4|[f=nut]pipe:" | ffplay pipe:

  It doesn’t throw errors, records "before.mp4" but neither "after.mp4" nor the preview are working. Surely I forgot something in the syntax.

• Revision 31020 : max = 255, ça bloquait la création de la table sur certaine base

  20 août 2009, par vincent@… — Log

  max = 255, ça bloquait la création de la table sur certaine base