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• Use FFMPEG in OpenCV without internet connection

  17 novembre 2022, par Henalds

  I am creating a project in c++ with QtCreator (5.14.1, MingGW compiler) and trying to use OpenCv (3.4.16) to read video files. I have tried many files of standard formats and codecs (H.264, yuv420, .mov etc). However, no matter what I try, VideoCapture() always silently fails. It doesn’t crash or show any error code, instead isOpened() is just always false.

  


  I think the cause is that I am building opencv (via this tutorial https://wiki.qt.io/How_to_setup_Qt_and_openCV_on_Windows) without internet connection (I cannot have internet connection on this machine, so please do not ask me to) and therefore it can’t download the FFMPEG libraries during this process. I have been looking everywhere for information about how to download the FFMPEG libraries for opencv directly but I haven’t had any luck.

  


  Can someone please explain what libraries I need to download and how opencv goes about looking for them ? because at the moment I don’t know what I need, nor where to put them, and I cant find any information on the topic.

  


  Or, can someone explain why calling VideoCapture(“video.mov”, cv::CAP_ANY) doesn’t have any effect ? (despite being able to play the video easily in VLC, MediaPlayer etc).

  


  Code :
`

  


  int main()
{
    VideoCapture cap(“C://video.mov”);
    //VideoCapture cap(“C:/video.mov”);
    //VideoCapture cap(“C:\video.mov”);
    //VideoCapture cap(“C:\video.mov”);

    if (!cap.isOpened()){
        cout << "Error opening video stream or file"<< endl;
        return -1;
    }
}


  


  `

  


  I have tried downloading some ffmpeg DLLs and the EXEs and adding them to the PATH, no success. I have also tried downloading the shared GPL of ffmpeg (that comes with the libs and include) and added them my .pro file but no change in VideoCapture behavior.

  


  I have also tried moving the opencv_ffmpeg_64.dll (found in opencv/build/bin) to my executable directory but that didn't fix anything.

  


• ffmpeg playback problems when concatenating 2 gif of same type into 1 file without re-encoding [closed]

  15 juin 2022, par kite

  I see this information here, but it didn't help :
https://stackoverflow.com/a/11175851/11361179

  


  Given 2 gif files 1.gif 2.gif of same type, I want to merge them into a single .gif file and stream without re-encode. Following the instructions here https://trac.ffmpeg.org/wiki/Concatenate : I make a list in windows called mylist with the names, then I concatenate using the below. The file created only runs correctly in ffplay though, anywhere else it will simply repeat the first 1.gif over and over and not play the other.

  


  (for %i in (*.gif) do @echo file '%i') > mylist.txt
ffmpeg -f concat -i mylist.txt -c copy output.gif


  


  I downloaded the newest version of ffmpeg and it didn't change anything. I also tried : ffmpeg -i "concat:input1|input2" -codec copy output.mkv. No change. I need the output to play properly in programs other than ffplay.

  


  The file generated is of the same length as 1.gif rather than the combined, and seeking has issues in ffplay, so there seems to be an issue with the output not having the right metadata for proper playback. All 3 files are the exact same in ffprobe except their bitrate value, as they should be since both gif were taken in same program with same settings of same length....with the exception that the third gif should be the combined length which it isn't. Removing -c copy and re-encoding works but generates a gif 50x the size of horrid quality.

  


• node.js live streaming ffmpeg stdout to res

  24 mai 2014, par blasteye

  I want node.js to convert an extremly long audio file to mp3, and the second data is available on stdout, node.js should send it to the client for them to play.

  I’ve written the following, and while it works, the html5 audio/video tag waits until ffmpeg is 100% done transcoding, where-as I want to start playing the video while ffmpeg is doing its thing.

  var ffmpeg = childProcess.spawn('ffmpeg', [
  
     '-i', params.location, //location of the specified media file
  
     '-f', 'mp3',
  
     'pipe:1'
  
   ]);
  
   res.writeHead(200, {
  
     'Content-Type': 'audio/mp3'
  
   });
  
   ffmpeg.stdout.pipe(res);

  EDIT 1 : Not sure why, but if params.location points to a movie everything seems to work. But if its an audio file, ffmpeg doesn’t seem to be outputting to stdout until its 100% converted.

  EDIT 2 : Turns out that you can’t dump an mp4 file to stdout due to the fact that mp4 files are non Causal (http://en.wikipedia.org/wiki/Causal_system). THerefore if you use webm it works. Just make sure to compile ffmpeg with webm support (for homebrew users : brew install ffmpeg —with-vpx —with-vorbis ).

  I’ve uploaded a github gist showing two functions to send live mp3/webm transcodes : https://gist.github.com/cobookman/c1a9856a4588496b021a