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• Getting motion vector side data from ffmpeg from the compressed domain (without performing full decode)

  4 mai 2019, par John Allard

  The ffmepg source code comes with a file extract_mvs.c which is quite helpful in showing how to extract the motion vectors from an h264-encoded video file. Unfortunately, it performs a full decode before getting the motion vectors from the side data of the stream.

  One should be technically able to (as was confirmed by FFMPEG developer Carl Eugen in this thread https://ffmpeg.org/pipermail/libav-user/2016-December/009913.html) get the motion vectors from the compressed domain before performing a full decode step. This would obviously be less computationally intensive as one wouldn’t have to perform a full decode.

  Is it possible to do this with ffmpeg/libavcodec as is ? I know Carl said that it wasn’t possible but that was 3 years ago. If it is not currently possible, does anyone have any hints on how one would go about modifying the ffmpeg source code to do this ?

  Thanks

• avutil/hwcontext_videotoolbox : BGRA should be full range

  3 janvier 2023, par Zhao Zhili

  avutil/hwcontext_videotoolbox : BGRA should be full range
  HEVC alpha layer encoding is broken without the patch.
  Signed-off-by : Zhao Zhili <zhilizhao@tencent.com>
  

  • [DH] libavutil/hwcontext_videotoolbox.c

• Movie from PNGs using FFmpeg and Python - rgb24 vs yuv420p

  7 février 2016, par Eilam G

  I wrote a little Python program to grab a bunch of PNGs and render them into a movie using the FFmpeg command line. The PNGs are read into [X*Y*3] numpy arrays (ignoring the alpha channel), new frames are added via interpolation, and the data is fed into FFmpeg as a pipe and saved as an mp4.

  The files play fine in VLC on Windows, but don’t work in iMovie on a Mac. I think it might have to do with most programs expecting H264 videos to be in the YUV420P color space, which my movies aren’t. I’ve tried changing the ffmpeg command -pix_fmt from rgb24 to yuv420p, but no go.

  Relevant Python code attached below.

  def init_vars(args):
  
      global log_file, file_names, command, num_int_frames, num_files, silent
  
  
  
      file_names = glob('./*.png')
  
      num_files = len(file_names)
  
  
  
      if args.log:
  
          log_file = 'bmmp.log'
  
      else:
  
          log_file = os.devnull
  
  
  
      silent = args.silent
  
  
  
      frames_per_second = args.fps
  
      wanted_movie_length = args.length
  
      movie_file_name = args.name + '.mp4'
  
  
  
      num_int_frames = round((frames_per_second * wanted_movie_length - 1) / (num_files - 1) - 1)
  
  
  
      if sys.platform == 'win32':
  
          ffmpeg_bin = 'ffmpeg.exe'
  
      else:
  
          ffmpeg_bin = 'ffmpeg'
  
  
  
      command = [ffmpeg_bin,
  
                 '-y', # (optional) overwrite output file if it exists
  
                 '-f', 'rawvideo',
  
                 '-vcodec','rawvideo',
  
                 '-s', '1280x720', # size of one frame
  
                 '-pix_fmt', 'rgb24',
  
                 '-r', str(frames_per_second), # frames per second
  
                 '-i', '-', # The input comes from a pipe
  
                 '-an', # Tells FFMPEG not to expect any audio
  
                 movie_file_name]

  Cheers,
  Eilam