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• FFmpeg av_read_frame returns a size but no data ?

  11 décembre 2013, par tommed

  I have written some C code to access ffmpeg and wrapped it in a C++/CLI (.NET managed) class. The program fetches a live video stream and extracts frames and converts them to PNG files.

  Unfortunately the images that are saved to disk are always black (opening them in Notepad++ shows that they are full of nulls).

  I am using the assemblies aformat/codec-55.dll and the development headers and libs for compilation from ffmpeg-20131120-git-e502783-win64-dev. The whole project is compiled using Managed C++ (Cpp/cli) .NET 4.0 for 64-bit.

  After some investigation the problem appears to be that av_read_frame fills the AVPacket->size value correctly, but the AVPAcket->data is always null. When the frame is finished (got==1) then the data for the AVFrame is just a matrix of nulls. :(

  Here is the code :
  Example code (sorry, but it didnt paste well into SO)

  I think the problem is at line 34 when the packet is returned like so :
  

  Please, how can I get this to work ? What have I done wrong ?

• VideoCapture always returns False in Python OPENCV [Linux]

  26 octobre 2017, par Daniyal Shahrokhian

  Every time that I use VideoCapture trying to access the frames from a video file, the return value (ret) is false. See the sample code below :

  cap = cv2.VideoCapture('asd.mkv')
  
      vid = []
  
      while True:
  
          ret, img = cap.read()
  
          if not ret: # Always happens
  
              break
  
          vid.append(cv2.resize(img, (171, 128)))

  I have already tried absolutely everything I could find today by googling, including the OpenCV guide and this long issue on Github. Also, I read some solutions involving moving ffmpeg dll files, but that only was in the case of Windows.

  Any ideas ? Because I defenitely ran out of them.

• VideoCapture always returns False in Python OPENCV [Linux]

  7 avril 2017, par Daniyal Shahrokhian

  Every time that I use VideoCapture trying to access the frames from a video file, the return value (ret) is false. See the sample code below :

  cap = cv2.VideoCapture('asd.mkv')
  
      vid = []
  
      while True:
  
          ret, img = cap.read()
  
          if not ret: # Always happens
  
              break
  
          vid.append(cv2.resize(img, (171, 128)))

  I have already tried absolutely everything I could find today by googling, including the OpenCV guide and this long issue on Github. Also, I read some solutions involving moving ffmpeg dll files, but that only was in the case of Windows.

  Any ideas ? Because I defenitely ran out of them.