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• OS X Mavericks broken OpenCV VideoCapture frame numbers ?

  20 novembre 2013, par MattP

  Fresh install of Mavericks, managed to install ffmpeg via homebrew and compile the latest OpenCV at time of writing. My code is written in Python (though this shouldn't affect this issue) and basically navigates a video file using the VideoCapture class with the properties set frame position and get current position.

  The issue is, on prior versions of OS X, various Linux and Windows boxes, this works without issue. Each frame number is of the format i.0, where i is the current frame number (why it is a float anyway is confusing, alas it's how OpenCV gives it back). However, on this setup I get frame progressions like :

  0.0 -> 1.3972597329753 -> 2.999999999 -> 3.9999999 -> 5.0 -> 6.252323552

  This makes using the properties for getting / setting the current frame position impossible and, as one would imagine, causes the automated playback thereof to fail. I can only assume ffmpeg is reporting things odd such that OpenCV is incorrectly calculating frames, but have been unable to rectify this.

  Has anyone any insight into the problem, and/or possible solution ? Thanks.

• v4l2 : setting device parameters early

  2 janvier 2014, par Federico Simoncelli

  v4l2 : setting device parameters early
  Setting the v4l2 device parameters may influence video properties
  
  such as width, height and pixel format.
  
  This patch moves v4l2_set_parameters to be executed before fetching
  
  the video properties, avoiding errors such as (pal vs ntsc) :
   The v4l2 frame is 829440 bytes, but 691200 bytes are expected
  Signed-off-by : Federico Simoncelli <fsimonce@redhat.com>
  
  Signed-off-by : Michael Niedermayer <michaelni@gmx.at>
  

  • [DH] libavdevice/v4l2.c

• FFMpeg compress with wrong frame rate while sending raw pixel data from java application

  11 janvier 2014, par Marco

  I am trying to create a compressed h264 video file from a series of raw rgb24 pixel data.
  Pixel data is streamed from java application to ffmpeg.exe which is its subprocess.

  For testing purposes I've created a byte array with rgb24 pixel data and then sent it to ffmpeg's input stream.

  Everything works fine except one issue.

  It looks like the frame rate of the encoded file is wrong.
  FFMpeg should compress the video file with 30fps rate (-r 30).

  I am sending 30*20 frames. So I supposed to get a video file with length of 20 seconds.
  But instead of that the video file's length is 24 seconds.
  When I check the compressed file properties, it shows 30fps as expected.

  It looks like the ratio between expected and actual frame rate is 5 to 6 (20 —> 24,30 —> 36).
  I have also tried to stream rgb32 pixel data but I had the same results.

  code :

      ByteArrayOutputStream byteArray = new ByteArrayOutputStream();
  
      //building the frame
  
      for (int i=0; i<1024*800; i++)
  
      {
  
          byteArray.write(i&0xFF);    //r
  
          byteArray.write(i&0xFF);    //g
  
          byteArray.write(i&0xFF);    //b
  
      }
  
  
  
      byte pixelData[] = byteArray.toByteArray();
  
  
  
      File ffmpeg_output_msg = new File("ffmpeg_output_msg.txt");
  
  
  
      ProcessBuilder pb = new ProcessBuilder("ffmpeg.exe","-vcodec","rawvideo","-f","rawvideo","-pix_fmt","rgb24","-s","1024x800","-i","pipe:0","-r","30","-y","-c:v","libx264","out.mkv");
  
  
  
      pb.redirectErrorStream(true);
  
      pb.redirectOutput(ffmpeg_output_msg);
  
  
  
      Process p = pb.start();
  
      OutputStream ffmpegInput = p.getOutputStream();
  
  
  
      //30fps, 20secs
  
      for(int i=0;i<30*20;i++)
  
      {
  
          ffmpegInput.write(pixelData);
  
      }
  
      ffmpegInput.flush();
  
      ffmpegInput.close();

  Any ideas what could be the origin of that problem ?
  Thanks in advance

  UPDATE
  I have changed the frame rate to other values and it looks like the ffmpeg completely ignores that parameter. It always uses 25fps.
  using "-r 30" effects only on the data that's written in file's properties
  (Video : MPEG4 Video (H264) 1024x800 30fps [Video])

  Is it possible that there are two different framerates in the encoded file ?