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• Get Total number of frames and FPS faster than with OpenCV library in C++

  20 juillet 2018, par daniels_pa

  I need to check which video can be analyzed and which cannot given the total number of frames in a video and the fps of the video. I created a c++ program to do the checking. Analyzing each video is not an option since analyzing is time consuming.

  I used the OpenCV library for starters :

      cv::VideoCapture vid_to_analyze;
  
      vid_to_analyze.open( me_vid.vid_path.string() );
  
      me_vid.total_frames= static_cast<int>(vid_to_analyze.get(CV_CAP_PROP_FRAME_COUNT));
  
      me_vid.fps=vid_to_analyze.get(CV_CAP_PROP_FPS);
  
  
  
      if (!vid_to_analyze.isOpened())
  
      {
  
          std::cout &lt;&lt; "Skipping vid: "&lt;&lt; me_vid.vid_path.string()&lt;&lt;", couldn't open it" &lt;&lt; std::endl;
  
      }
  
      if (me_vid.fps != me_vid.fps || me_vid.fps &lt;= 0)
  
      {
  
          std::cout &lt;&lt; "For video " &lt;&lt; me_vid.vid_path.string() &lt;&lt; std::endl;
  
          std::cout &lt;&lt; "FPS of the video file cannot be determined, assuming 30"&lt;&lt; std::endl;
  
          me_vid.fps = 30;
  
      }
  
  
  
      vid_to_analyze.release();
  
  </int>

  However when debugging it becomes painfully slow (the program is faster running without the debugger attached but still very slow given the number of videos it needs to cover). I think that has something to do with 4 threads being created and deleted each time a video is opened (released).

  How to get total number of frames and fps in a faster manner ( without actually creating 4 threads !!) if i am not interested in actually grabbing frames from the video just the number of frames and fps.

  Is there a way to use ffmpeg library from c++, would that be faster and where to start ?

  EDIT : Valgrind seems to agree since (Ir=)91.66% of time spend in the vid_to_analyze.open phase

• How do I set total file duration generating a piped mp4 with ffmpeg ?

  9 avril 2021, par Mikhail Novikov

  My task is to generate (by piping, so that a file can be played at the same time with generation) an mp4 file which is a part of a larger file, with the result looking like a static file link, being seekable before it fully loads (i.e. supporting range headers).

  


  Here is how I do it now :

  


  ffmpeg -ss $1 -i teststream_hls1_replay.mp4 -t $2 -timecode $3 \
       -codec copy -movflags frag_keyframe+faststart -f mp4 pipe:1


  


  Result is OK (video starts from the right point) except the player does not see the total duration of a file so a controlbar looks weird, and seeking isn't possible properly, just because the controlbar jumps all the time.

  


  How do I indicate to ffmpeg that it has to set moov atom to contain right duration ?

  


  Basically the question boils down to : how do I force set some arbitrary duration of file in a moov atom, when I am generating a fragmented mp4 ? ffmpeg will not get know how long will it be, so explainably it can't do it itself, but I know... is there a command line parameters to specify a 'forced duration' ?

  


• ffmpeg - how can I scale the video while perserving the total frames in it ?

  14 juin 2018, par choz

  I have a video with a size of 515 x 755, that I would like to resize proportionally to 206 x 302 (so, factor is 2.5).

  In this video, I have the total of 588 frames, and I use ffmpeg to scale it, with this command.

  ffmpeg -i video.mp4 -vf scale=206:-1 xRotation_206.mp4

  And I use this to check how many frames are there in the video, based on this answer.

  ffmpeg -i video.mp4 -map 0:v:0 -c copy -f null - 2>&1 | awk '/frame=/ {print $2}'

  The original video frames is good (588 frames). But, after resizing with the command above, my converted video now has 604 frames.

  The question is, how can I resize the video by preserving the total frames in it ? It doesn’t really have to be ffmpeg approach, and any suggestion is appreciated.

  Here’s the sample video that I use : Link