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• Is there any control property to fix video playback speed problem when using ffmpeg to decode in Qt platform ?

  16 février 2019, par SoloWang

  I want to play local video file in Qt platform using ffmpeg to decode.Everything is OK except that play speed is as twice as normal.
  The first thing I think about is that there must be a sampling frequency involved.But to be a new to ffmpeg,I don’t know how to fix this problem.
  Above is my code to read frame,is anyone can tell me what’s wrong with the code ?

  void VideoThread::run()
  
  {
  
      m_pInFmtCtx = avformat_alloc_context(); //ini struct
  
      char path[] = "d:/test.mp4";
  
      // open specific file
  
      if(avformat_open_input(&m_pInFmtCtx, *path, NULL, NULL)){ 
  
      {
  
          qDebug()<<"get rtsp failed";
  
          return;
  
      }
  
      else
  
      {
  
          qDebug()<<"get rtsp success";
  
      }
  
  
  
  
  
      if(avformat_find_stream_info(m_pInFmtCtx, NULL) < 0)
  
      {
  
          qDebug()<<"could not find stream information";
  
          return;
  
      }
  
      int nVideoIndex = -1;
  
      for(int i = 0; i < m_pInFmtCtx->nb_streams; i++)
  
      {
  
          if(m_pInFmtCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO)
  
          {
  
              nVideoIndex = i;
  
              break;
  
          }
  
      }
  
      if(nVideoIndex == -1)
  
      {
  
          qDebug()<<"could not find video stream";
  
          return;
  
      }
  
  
  
      qDebug("---------------- File Information ---------------");
  
      m_pCodecCtx = m_pInFmtCtx->streams[nVideoIndex]->codec;
  
      m_pCodec = avcodec_find_decoder(m_pCodecCtx->codec_id);
  
      if(!m_pCodec)
  
      {
  
          qDebug()<<"could not find codec";
  
          return;
  
      }
  
      //start Decoder
  
      if (avcodec_open2(m_pCodecCtx, m_pCodec, NULL) < 0) {
  
          qDebug("Could not open codec.\n");
  
          return;
  
      }
  
  
  
  
  
      //malloc space for stroring frame
  
      m_pFrame     = av_frame_alloc();
  
      m_pFrameRGB  = av_frame_alloc();
  
      m_pOutBuf = (uint8_t*)av_malloc(avpicture_get_size(AV_PIX_FMT_RGB32, m_pCodecCtx->width, m_pCodecCtx->height));
  
      avpicture_fill((AVPicture*)m_pFrameRGB, m_pOutBuf, AV_PIX_FMT_RGB32, m_pCodecCtx->width, m_pCodecCtx->height);
  
  
  
      //for color switch,from YUV to RGB
  
      struct SwsContext *pImgCtx = sws_getContext(m_pCodecCtx->width, m_pCodecCtx->height, m_pCodecCtx->pix_fmt,
  
                                                  m_pCodecCtx->width, m_pCodecCtx->height, AV_PIX_FMT_RGB32, SWS_BICUBIC, NULL, NULL, NULL);
  
  
  
  
  
      int nSize = m_pCodecCtx->width * m_pCodecCtx->height;
  
      m_pPacket = (AVPacket *)av_malloc(sizeof(AVPacket));
  
      if(av_new_packet(m_pPacket, nSize) != 0)
  
      {
  
          qDebug()<<"new packet failed";
  
      }
  
  
  
      //isInterruptionRequested is a flag,determine whether the thread is over
  
      // read each frame from specific video file
  
      while (!isInterruptionRequested())
  
      {
  
          int nGotPic = 0;
  
          if(av_read_frame(m_pInFmtCtx, m_pPacket) >= 0)
  
          {
  
              if(m_pPacket->stream_index == nVideoIndex)
  
              {
  
                  //avcodec_decode_video2()transform from packet to frame
  
                  if(avcodec_decode_video2(m_pCodecCtx, m_pFrame, &nGotPic, m_pPacket) < 0)
  
                  {
  
                      qDebug()<<"decode failed";
  
                      return;
  
                  }
  
                  if(nGotPic)
  
                  {   // transform to RGB color
  
                      sws_scale(pImgCtx, (const uint8_t* const*)m_pFrame->data,
  
                                m_pFrame->linesize, 0, m_pCodecCtx->height, m_pFrameRGB->data,
  
                                m_pFrameRGB->linesize);
  
                      // save to QImage,for later use
  
                      QImage *pImage = new QImage((uchar*)m_pOutBuf, m_pCodecCtx->width, m_pCodecCtx->height, QImage::Format_RGB32);
  
                  }
  
  
  
              }
  
          }
  
  
  
          av_free_packet(m_pPacket);
  
          msleep(5);
  
      }
  
      exec();
  
  }

• FFMPEG libav : Control when video frame is captured

  24 mai 2019, par Michael Murray

  I am using a the libav library to record video from a CSI camera on a raspberry pi. I can successfully record the camera data to a file, however I now want to synchronise the frames between two cameras. I’ve setup a mechanism to synchronise when frames are captured, however I assumed when I called av_read_frame(input_format_context, &packet), that the next frame would be captured. However, it turns out that the video is streamed into some buffer, and when the buffer is full, it waits until the av_read_frame method is called which removes data from the buffer, thus allowing another frame to be captured.

  This is not the behaviour I want, as I need to control exactly when the frame is captured. Is there a mechanism in libav that I could use to produce this kind of behaviour ? Or am I going to have to use a different library for capturing video from the CSI device ?

• How to fix : GUI application who control a simple command of ffmpeg with python

  9 octobre 2019, par maoca

  I want to make a graphic application with only 2 buttons (start / stop) that allows to launch a subprocess (start) and stop it (stop).
  (I’m using Python3, PyQt5 and ffmpeg)
  The process captures the screen in a video and save it to an mp4 using the ffmpeg command to launch the POpen command.
  To make a clean output of the command, ffmpeg uses ’q’ that I write by stdin.

  In a simple script it works for me but I can’t get it to work within the buttons.

  My knowledge is very basic and as much as I look for information I do not understand what I am doing wrong, I appreciate any comments that let me move on.

  This is my code :

  import sys
  
  import subprocess
  
  from PyQt5.QtWidgets import QApplication, QWidget, QPushButton
  
  
  
  class Ventana(QWidget):
  
  
  
      def __init__(self):
  
          super().__init__()
  
          # Button 1
  
          pybutton = QPushButton('REC', self)
  
          pybutton.clicked.connect(self.clickMethodB1)
  
          pybutton.resize(50, 32)
  
          pybutton.move(50, 50)
  
  
  
          # BOTON 2
  
          pybutton = QPushButton('STOP', self)
  
          pybutton.clicked.connect(self.clickMethodB2)
  
          pybutton.resize(100, 32)
  
          pybutton.move(150, 50)
  
  
  
  
  
          self.initUI()
  
  
  
  
  
      def initUI(self):
  
          self.setGeometry(300, 300, 300, 220)
  
          self.setWindowTitle('FFMPEG')
  
          self.move(800, 400)
  
          self.show()
  
  
  
      def clickMethodB1(self):
  
          global ffmpeg
  
          filename_mp4 = 'c://tmp//output.mp4'
  
          print('REC')
  
  
  
          command = 'ffmpeg -f dshow -i video="screen-capture-recorder" '+ filename_mp4
  
  
  
          ffmpeg = subprocess.Popen(command, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE,                                                    encoding='utf-8', shell=True)
  
  
  
  
  
      def clickMethodB2(self):
  
  
  
          print('STOP')
  
          ffmpeg.stdin.write(str('q'))            
  
  
  
  
  
  if __name__ == '__main__':
  
      app = QApplication(sys.argv)
  
      ex = Ventana()
  
      sys.exit(app.exec_())