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• What's the difference with crf and qp in ffmpeg ?

  18 novembre 2016, par Nova

  After browsing around Google, I’ve came across this page about h264 encoding and discovered about qp. https://trac.ffmpeg.org/wiki/Encode/H.264

  My questions are : What are the differences with crf and qp ? Is it better to use qp over crf overall, or is it only if for using qp 0 for best lossless ? Does qp have a known sensible setting if it’s preferred ? So far, I know crf has the default value of 23 while 18 is a sensible preferred increase in quality, although I don’t understand why 18 wouldn’t be default if better sensible lossless. Lastly, would changing either of them cause incompatibility with non-ffmpeg players or just qp ?

  I’m converting from webm to mp4 by the way.

  I was going to test crf 23 and 18 and pick which is best but I can’t seem to find any concrete information on this comparison or about qp.

• avformat/movenc : Check first DTS similar to dts difference

  9 septembre 2016, par Michael Niedermayer

  avformat/movenc : Check first DTS similar to dts difference
  Fixes assertion failure
  
  Fixes : b84b53855a0b74560e64c6f45f505a13/signal_sigabrt_7ffff6ae7c37_3837_ef4e243ea5b4fa8d0becf4afe9166604.avi
  Found-by : Mateusz "j00ru" Jurczyk and Gynvael Coldwind
  
  Signed-off-by : Michael Niedermayer <michael@niedermayer.cc>
  

  • [DH] libavformat/movenc.c

• Difference between ffmpeg and avconv for rawvideo image2pipe

  15 août 2016, par NHDaly

  I’m not sure why, but avconv does not seem to be piping raw video like I would expect.

  I’m trying pipe a video from ffmpeg into python (eventually I want to read from x11grab, not a video file). It works just fine on my Macbook using ffmpeg, but when I use avconv on Debian Jessie, the stream cuts off early !

  Here’s my basic python, which is following this guide :

  input_resolution_shape = (1280,800,3)
  
  input_bytes = reduce(mul, input_resolution_shape, 1)
  
  print input_bytes
  
  
  
  # Prints 3072000
  
  
  
  import subprocess as sp
  
  command = [ FFMPEG_BIN,   # This is either "avconv" or "ffmpeg".
  
              '-i', 'test_video.mp4',
  
              '-f', 'image2pipe',
  
              '-pix_fmt', 'rgb24',
  
              '-vcodec', 'rawvideo', '-']
  
  pipe = sp.Popen(command, stdout = sp.PIPE, bufsize=10**8)
  
  
  
  import numpy
  
  for _ in range(100):  # read 100 frames
  
      # read 1280*800*3 bytes (= 1 frame)
  
      raw_image = pipe.stdout.read(input_bytes)
  
      # transform the byte read into a numpy array
  
      image = numpy.fromstring(raw_image, dtype='uint8')
  
      if image.size != 0:
  
          print image.size
  
  
  
          # Prints 1015808

  On the mac, the image.size printed is the same as the input_bytes, 3072000. But on debian, it’s 1015808. Any ideas why that could be happening ?

  Interestingly, 3072000/1015808 is just about 3 :

  In [1]:    3072000./1015808.
  
  Out[1]:    3.024193548387097