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• What is video timescale, timebase, or timestamp in ffmpeg ?

  28 février 2020, par Please Help

  There does not seem to be any explanation online as to what these are. People talk about them a lot. I just want to know what they are and why they are significant. Using -video_track_timescale, how would I determine a number for it ? Is it random ? Should it be 0 ?

• How to tell if faststart for video is set using ffmpeg or ffprobe

  5 septembre 2022, par jsherk

  I am having trouble figuring out how to determine if faststart is set on an MP4 video.

  



  I understand that "moov atom" is the data that needs to be located at the beginning of the file for faststart to be enabled, instead of at the end of the file.

  



  I specifically want to use ffmpeg or ffprobe to determine if it has been moved to the beginning or not already.

  



  On a side note, I understand I can run the following command to move it from the end to the beginning (but I want to find out if it is already there) :

  



  ffmpeg -i infile.mp4 -map 0 -c:v copy -c:a copy -c:s copy -c:d copy -c:t copy -movflags +faststart outfile.mp4


  


• OpenCV's VideoCapture::open Video Source Dialog

  13 novembre 2015, par swtdrgn

  In my current project, when I call VideoCapture::open(camera device index) and the camera is in used by another program, it shows a Video Source dialog and returns true when I select a device that is already in use.

  However, in my [previous] experiment project, when I called VideoCapture::open(camera device index), it doesn’t show this dialog.

  I want to know what is causing the Video Source dialog to show and the program to behave differently from the experimental project.

  This is the source code to the experiment project :

  int main (int argc, char *argv[])
  
  {
  
  
  
      //vars
  
      time_duration td, td1;
  
      ptime nextFrameTimestamp, currentFrameTimestamp, initialLoopTimestamp, finalLoopTimestamp;
  
      int delayFound = 0;
  
      int totalDelay= 0;
  
  
  
      // initialize capture on default source
  
      VideoCapture capture;
  
      std::cout << "capture.open(0): " << capture.open(0) << std::endl;
  
      std::cout << "NOOO" << std::endl;
  
      namedWindow("video", 1);
  
  
  
      // set framerate to record and capture at
  
      int framerate = 15;
  
  
  
      // Get the properties from the camera
  
      double width = capture.get(CV_CAP_PROP_FRAME_WIDTH);
  
      double height = capture.get(CV_CAP_PROP_FRAME_HEIGHT);
  
  
  
      // print camera frame size
  
      //cout << "Camera properties\n";
  
      //cout << "width = " << width << endl <<"height = "<< height << endl;
  
  
  
      // Create a matrix to keep the retrieved frame
  
      Mat frame;
  
  
  
      // Create the video writer
  
      VideoWriter video("capture.avi",0, framerate, cvSize((int)width,(int)height) );
  
  
  
      // initialize initial timestamps
  
      nextFrameTimestamp = microsec_clock::local_time();
  
      currentFrameTimestamp = nextFrameTimestamp;
  
      td = (currentFrameTimestamp - nextFrameTimestamp);
  
  
  
      // start thread to begin capture and populate Mat frame
  
      boost::thread captureThread(captureFunc, &frame, &capture);
  
      // loop infinitely
  
      for(bool q=true;q;)
  
      {
  
          if(frame.empty()){continue;}
  
          //if(cvWaitKey( 5 ) == 'q'){ q=false; }
  
          // wait for X microseconds until 1second/framerate time has passed after previous frame write
  
          while(td.total_microseconds() < 1000000/framerate){
  
              //determine current elapsed time
  
              currentFrameTimestamp = microsec_clock::local_time();
  
              td = (currentFrameTimestamp - nextFrameTimestamp);
  
              if(cvWaitKey( 5 ) == 'q'){
  
                  std::cout << "B" << std::endl;
  
                  q=false;
  
                  boost::posix_time::time_duration timeout = boost::posix_time::milliseconds(0);
  
                  captureThread.timed_join(timeout);
  
                  break;
  
              }
  
          }
  
  
  
          // determine time at start of write
  
          initialLoopTimestamp = microsec_clock::local_time();
  
  
  
          // Save frame to video
  
          video << frame;
  
          imshow("video", frame);
  
  
  
          //write previous and current frame timestamp to console
  
          cout << nextFrameTimestamp << " " << currentFrameTimestamp << " ";
  
  
  
          // add 1second/framerate time for next loop pause
  
          nextFrameTimestamp = nextFrameTimestamp + microsec(1000000/framerate);
  
  
  
          // reset time_duration so while loop engages
  
          td = (currentFrameTimestamp - nextFrameTimestamp);
  
  
  
          //determine and print out delay in ms, should be less than 1000/FPS
  
          //occasionally, if delay is larger than said value, correction will occur
  
          //if delay is consistently larger than said value, then CPU is not powerful
  
          // enough to capture/decompress/record/compress that fast.
  
          finalLoopTimestamp = microsec_clock::local_time();
  
          td1 = (finalLoopTimestamp - initialLoopTimestamp);
  
          delayFound = td1.total_milliseconds();
  
          cout << delayFound << endl;
  
  
  
          //output will be in following format
  
          //[TIMESTAMP OF PREVIOUS FRAME] [TIMESTAMP OF NEW FRAME] [TIME DELAY OF WRITING]
  
          if(!q || cvWaitKey( 5 ) == 'q'){
  
              std::cout << "C" << std::endl;
  
              q=false;
  
              boost::posix_time::time_duration timeout = boost::posix_time::milliseconds(0);
  
              captureThread.timed_join(timeout);
  
              break;
  
          }
  
      }
  
  
  
      // Exit
  
      return 0;
  
  }