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• avcodec_decode_video2 fails to decode after frame resolution change

  10 avril 2021, par Krzysztof Kansy

  I'm using ffmpeg in Android project via JNI to decode real-time H264 video stream. On the Java side I'm only sending the the byte arrays into native module. Native code is running a loop and checking data buffers for new data to decode. Each data chunk is processed with :
  

  



  int bytesLeft = data->GetSize();
int paserLength = 0;
int decodeDataLength = 0;
int gotPicture = 0;
const uint8_t* buffer = data->GetData();
while (bytesLeft > 0) {
    AVPacket packet;
    av_init_packet(&packet);
    paserLength = av_parser_parse2(_codecPaser, _codecCtx, &packet.data, &packet.size, buffer, bytesLeft, AV_NOPTS_VALUE, AV_NOPTS_VALUE, AV_NOPTS_VALUE);
    bytesLeft -= paserLength;
    buffer += paserLength;

    if (packet.size > 0) {
        decodeDataLength = avcodec_decode_video2(_codecCtx, _frame, &gotPicture, &packet);
    }
    else {
        break;
    }
    av_free_packet(&packet);
}

if (gotPicture) {
// pass the frame to rendering
}


  



  The system works pretty well until incoming video's resolution changes. I need to handle transition between 4:3 and 16:9 aspect ratios. While having AVCodecContext configured as follows :

  



  _codecCtx->flags2|=CODEC_FLAG2_FAST;
_codecCtx->thread_count = 2;
_codecCtx->thread_type = FF_THREAD_FRAME;

if(_codec->capabilities&CODEC_FLAG_LOW_DELAY){
    _codecCtx->flags|=CODEC_FLAG_LOW_DELAY;
}


  



  I wasn't able to continue decoding new frames after video resolution change. The got_picture_ptr flag that avcodec_decode_video2 enables when whole frame is available was never true after that.
  
This ticket made me wonder if the issue isn't connected with multithreading. Only useful thing I've noticed is that when I change thread_type to FF_THREAD_SLICE the decoder is not always blocked after resolution change, about half of my attempts were successfull. Switching to single-threaded processing is not possible, I need more computing power. Setting up the context to one thread does not solve the problem and makes the decoder not keeping up with processing incoming data.
  
Everything work well after app restart.
  

  



  I can only think of one workoround (it doesn't really solve the problem) : unloading and loading the whole library after stream resolution change (e.g as mentioned in here). I don't think it's good tho, it will propably introduce other bugs and take a lot of time (from user's viewpoint).

  



  Is it possible to fix this issue ?

  



  EDIT :
  
I've dumped the stream data that is passed to decoding pipeline. I've changed the resolution few times while stream was being captured. Playing it with ffplay showed that in moment when resolution changed and preview in application froze, ffplay managed to continue, but preview is glitchy for a second or so. You can see full ffplay log here. In this case video preview stopped when I changed resolution to 960x720 for the second time. (Reinit context to 960x720, pix_fmt: yuv420p in log).

  


• OpenCV 4.5.2 takes a long time (>100ms) to retrieve a single frame from a webcam, C++ on Windows 10

  9 juin 2021, par Mustard Tiger

  I've been having a tough time getting my webcam working quickly with opencv. Frames take a very long time to read, (a recorded average of 124ms across 500 frames) I've tried on three different computers (running Windows 10) with a logitech C922 webcam. The most recent machine I tested on has a Ryzen 9 3950X, with 32gbs of ram ; no lack of power.

  


  Here is the code :

  


  cv::VideoCapture cap = cv::VideoCapture(m_cameraNum);&#xA;&#xA;// Check if camera opened successfully&#xA;if (!cap.isOpened())&#xA;{&#xA;    m_logger->critical("Error opening video stream or file\n\r");&#xA;    return -1;&#xA;}&#xA;&#xA;bool result = true;&#xA;result &amp;= cap.set(cv::CAP_PROP_FRAME_WIDTH, 1280);&#xA;result &amp;= cap.set(cv::CAP_PROP_FRAME_HEIGHT, 720);&#xA;&#xA;bool ready = false;&#xA;std::vector<string> timeLog;&#xA;timeLog.reserve(50000);&#xA;int i = 0;&#xA;&#xA;while (i &lt; 500)&#xA;{&#xA;    auto start = std::chrono::system_clock::now();&#xA;    &#xA;    cv::Mat img;&#xA;    ready = cap.read(img);&#xA;&#xA;    // If the frame is empty, break immediately&#xA;    if (!ready)&#xA;    {&#xA;        timeLog.push_back("continue");&#xA;        continue;&#xA;    }&#xA;&#xA;    i&#x2B;&#x2B;;&#xA;    auto end = std::chrono::system_clock::now();&#xA;    timeLog.push_back(std::to_string(std::chrono::duration_cast(end - start).count()));&#xA;}&#xA;&#xA;for (auto&amp; entry : timeLog)&#xA;    m_logger->info(entry);&#xA;&#xA;cap.release();&#xA;return 0;&#xA;</string>

  


  Notice that I write the elapsed time to a log file at the end of execution. The average time is 124ms for debug and release, and not one instance of "continue" after half a dozen runs.

  


  It doesn't matter if I use USB 2 or USB 3 ports (the camera is USB2) or if I run a debug build or a release build, the log file will show anywhere from 110ms to 130ms of time for each frame. The camera works fine in other app, OBS can get a smooth 1080@30fps or 720@60fps.

  


  Stepping through the debugger and doing a lot of Googling, I've learned the following about my system :

  


  

  • The backend chosen by default is DSHOW. GStreamer and FFMPEG are also available.

  


  • DSHOW uses FFMPEG somehow (it needs the FFMPEG dll) but I cannot use FFMPEG directly through opencv. Attempting to use cv::VideoCapture(m_cameraNum, cv::CAP_FFMPEG) always fails. It seems like Opencv's interface to FFMPEG is only capable of opening video files.

  


  • Microsoft really screwed up camera devices in Windows a few years back, not sure if this is related to my problem.

  


  


  Here's a short list of the fixes I have tried, most taken from older SO posts :

  


  

  • result &= cap.set(cv::CAP_PROP_FRAME_COUNT, 30) ; // Returns false, does nothing

  


  • result &= cap.set(cv::CAP_PROP_CONVERT_RGB, 0) ; // Returns true, does nothing

  


  • result &= cap.set(cv::CAP_PROP_MODE, cv::VideoWriter::fourcc('M', 'J', 'P', 'G')) ; // Returns false, does nothing

  


  • Set registry key from http://alax.info/blog/1693 that should disable the new Windows camera server.

  


  • Updated from 4.5.0 to 4.5.2, no change.

  


  • Asked device manager to find a newer driver, no newer driver found.

  


  


  I'm out of ideas. Any help ?

  


• avcodec_decode_video2 fails to decode after frame resolution change

  7 octobre 2016, par Krzysztof Kansy

  I’m using ffmpeg in Android project via JNI to decode real-time H264 video stream. On the Java side I’m only sending the the byte arrays into native module. Native code is running a loop and checking data buffers for new data to decode. Each data chunk is processed with :
  

  int bytesLeft = data->GetSize();
  
  int paserLength = 0;
  
  int decodeDataLength = 0;
  
  int gotPicture = 0;
  
  const uint8_t* buffer = data->GetData();
  
  while (bytesLeft > 0) {
  
      AVPacket packet;
  
      av_init_packet(&packet);
  
      paserLength = av_parser_parse2(_codecPaser, _codecCtx, &packet.data, &packet.size, buffer, bytesLeft, AV_NOPTS_VALUE, AV_NOPTS_VALUE, AV_NOPTS_VALUE);
  
      bytesLeft -= paserLength;
  
      buffer += paserLength;
  
  
  
      if (packet.size > 0) {
  
          decodeDataLength = avcodec_decode_video2(_codecCtx, _frame, &gotPicture, &packet);
  
      }
  
      else {
  
          break;
  
      }
  
      av_free_packet(&packet);
  
  }
  
  
  
  if (gotPicture) {
  
  // pass the frame to rendering
  
  }

  The system works pretty well until incoming video’s resolution changes. I need to handle transition between 4:3 and 16:9 aspect ratios. While having AVCodecContext configured as follows :

  _codecCtx->flags2|=CODEC_FLAG2_FAST;
  
  _codecCtx->thread_count = 2;
  
  _codecCtx->thread_type = FF_THREAD_FRAME;
  
  
  
  if(_codec->capabilities&CODEC_FLAG_LOW_DELAY){
  
      _codecCtx->flags|=CODEC_FLAG_LOW_DELAY;
  
  }

  I wasn’t able to continue decoding new frames after video resolution change. The got_picture_ptr flag that avcodec_decode_video2 enables when whole frame is available was never true after that.
  This ticket made me wonder if the issue isn’t connected with multithreading. Only useful thing I’ve noticed is that when I change thread_type to FF_THREAD_SLICE the decoder is not always blocked after resolution change, about half of my attempts were successfull. Switching to single-threaded processing is not possible, I need more computing power. Setting up the context to one thread does not solve the problem and makes the decoder not keeping up with processing incoming data.
  Everything work well after app restart.
  

  I can only think of one workoround (it doesn’t really solve the problem) : unloading and loading the whole library after stream resolution change (e.g as mentioned in here). I don’t think it’s good tho, it will propably introduce other bugs and take a lot of time (from user’s viewpoint).

  Is it possible to fix this issue ?

  EDIT :
  I’ve dumped the stream data that is passed to decoding pipeline. I’ve changed the resolution few times while stream was being captured. Playing it with ffplay showed that in moment when resolution changed and preview in application froze, ffplay managed to continue, but preview is glitchy for a second or so. You can see full ffplay log here. In this case video preview stopped when I changed resolution to 960x720 for the second time. (Reinit context to 960x720, pix_fmt: yuv420p in log).