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• ffmpeg, /dev/video0, -f decklink

  20 mars 2019, par Camille Goudeseune

  I’m trying to capture video from a PCI card, the Blackmagic DeckLink Mini Recorder, via ffmpeg, on a headless host running Ubuntu 18.04.2 LTS, hopefully with a command like

  ffmpeg -f decklink -i /dev/video0 ...

  How can I make that work ? I have two obstacles.

  No /dev/video0

  ffmpeg -i /dev/video0 ... fails : /dev/video0: No such device or address.
  v4l2-ctl --list-devices fails with the same error message.

  I built /dev/video0, and it looks okay :

  mknod /dev/video0 c 81 0
  
  chown root.video /dev/video0
  
  chmod g+rw /dev/video0

  To compare this file with a working one, I ran strace cat /dev/video0 on this host, and on another host (Ubuntu 14) with a working /dev/video0. The outputs began to differ here (good, then bad) :

  fstat(1, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
  
  open("/dev/video0", O_RDONLY)           = 3  
  
  fstat(3, {st_mode=S_IFCHR|0660, st_rdev=makedev(81, 0), ...}) = 0
  
  fadvise64(3, 0, 0, POSIX_FADV_SEQUENTIAL) = 0
  
  ----
  
  
  
  fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
  
  openat(AT_FDCWD, "/dev/video0", O_RDONLY) = -1 ENXIO (No such device or address)

  So /dev/video0 is broken at a level lower than ffmpeg or v4l2 or even cat.

  On Ubuntu 14, man 8 MAKEDEV suggests that the error message means that "the kernel does not have the driver configured or loaded."

  This Ubuntu 18 host lacks that manpage, but it does have a few /snap/core/*/sbin/MAKEDEV, all the same, so I tried

  /snap/core/6350/sbin/MAKEDEV -n -v video

  It would have created over a hundred devices videoXX, radioXX, vtxXX, vbiXX. Those devices didn’t exist yet, so it seemed harmless to try it.

  rm /dev/video0; /snap/core/6350/sbin/MAKEDEV video

  That rebuilt /dev/video0, but "No such device" remains, from cat or ffmpeg.

  No decklink

  ffmpeg -f decklink ... fails with Unknown input format: 'decklink'.

  Neither black nor deck nor link is mentioned by ffmpeg -devices (fbdev, lavfi, oss, v4l2) and ffmpeg -formats (about 350), either for Ubuntu’s own version 3.4.4-0ubuntu0.18.04.1, or for version N-93330-g7ff89574c7 compiled from source on 2019 Mar 13 :

  git clone https://git.ffmpeg.org/ffmpeg.git ffmpeg
  
  cd ffmpeg
  
  ./configure --enable-nonfree --disable-doc --disable-w32threads --enable-pthreads

  (Although ./configure --help mentions --enable-decklink, using that yielded "ERROR : DeckLinkAPI.h not found." updatedb && locate DeckLinkAPI.h finds no file with that name, either.)

  The DeckLink PCI card is recognized by hwinfo and lspci.

  lsmod reports the loaded modules blackmagic and blackmagic_io.

  Maybe the PCI card is installed ok, but ffmpeg just can’t reach it because I can’t configure it for that.

  Edit : Rebooting didn’t fix anything.

• Overlay circular video with transparency with maskedmerge

  15 juillet 2017, par cgenco

  I have a square video from Snap Spectacles (1088x1088) that I want to overlay on itself zoomed in and blurred.

  Example input frame :

  

  Generated zoomed in and blurred background :

  

  Desired output :

  

  I think I can do this with ffmpeg’s maskedmerge, but I’m having trouble finding examples.

  There’s an example of maskedmerge that merges two videos of the same size and dynamically removes a green screen, and another that merges videos with transparency.

  Here’s the closest I’ve been able to get :

  ffmpeg -i background.jpg -vf "movie=input.jpg[inner];[in][inner] overlay=#{offset}:0 [out]" -c:a copy output.jpg

  

  tl ;dr : given the first two frames, how could I generate the third frame (as video) ?

• OpenCV cannot connect to video stream - lack of some codec ?

  25 février 2016, par Chris92

  I use application IPCamera on my mobile phone with Android to output (share) video image from it’s camera to LAN. I can access it on PC browser - that is ok.

  However, I want to make OpenCV capture this video stream from IP address by typing

  VideoCapture cap("http://admin:admin@192.168.0.11:8081/?action=stream?dummy=param.mjpg");
  
   while( cap.isOpened() )
  
   {
  
      Mat frame;
  
      if ( ! cap.read(frame) )
  
          break;
  
  
  
      cout << "Connected!!";
  
  
  
      imshow("lalala",frame);
  
      int k = waitKey(10);
  
      if ( k==27 )
  
          break;
  
  }

  and i got error :

  

  Actual codec, which is used by phone is mjpeg (i read it from application on my mobile). I don’t know if OpenCV supports this, but is that about mobile application uses some kind of unique codec, or my PC lacks it, or maybe C++/OpenCV code is wrong ?