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• FFmpeg/X264 : Split video mid-GOP without reencoding entire stream

  10 juillet 2012, par yoda_alex

  I've got an H264 video (Stored in an MP4 file). Each GOP is approx 10s long. I want to trim the first couple of seconds off the video, which means I need to split the first GOP. Is there a way to do this without re-encoding the entire video ?

  I've got FFmpeg and x264 available. I'm happy to use either the FFmpeg command line, or my own program linked against ffmpeg of x264 libraries.

  Thanks in advance !

• Gallery of VP8 Encoding Naivete

  15 octobre 2010, par Multimedia Mike — VP8

  I’ve been toiling away as a multimedia technology generalist for so long that it’s easy for me to forget that not everyone is as versed in the minutiae of the domain as I am. But I recently experienced what it’s like to be such an outsider when I posted about my toy VP8 encoder, expressing that it’s one of the hardest things I have ever tried to do. I heard of from number of people who do have extensive experience in video encoding, particularly with the H.264 and VP8 codecs. Their reactions were predictable : What’s so hard ? Look, you might be a little too immersed in the area to really understand a relative beginner’s perspective.

  And to all the people who suggested that I should get the encoder into FFmpeg ASAP : Are you crazy ?! Did you see what the first pass of the encoder produced ? Do you have lower standards than even I do ?

  
  
  

  Not Giving Up
  I worked a little more on the toy encoder. Remember that the above image is what I’m hoping to encode somewhat faithfully for this experiment. In my first pass, I attempted vertical prediction for all planes. For my next pass, I forced the chroma planes to mid-level (which results in a greyscale image) and played with the 16×16 luma prediction modes. When implementing an extremely naive algorithm to decide which 16×16 prediction mode would be the best for a particular block, this is what the program produced :

  
  
  

  For fun, here is what the image encodes to when forcing various prediction modes :
  

  I think the DC-only prediction mode actually looks a little better than the image that the naive algorithm produced :

  
  
  

  Vertical 16×16 prediction, similar to the image from the last post (just in black and white) :
  

  
  
  

  Horizontal 16×16 prediction :
  

  
  
  

  This is the 16×16 prediction mode unique to VP8, the TrueMotion mode (based on On2/Duck’s very first video codec) :
  

  
  
  

  Wow, these encodings really bring down the cheerful tone of the original image.

  Next Steps
  I have little reason to believe that I am encoding and subsequently reconstructing the image correctly (i.e., error is likely propagating through the entire encoding). If I have time, the next step is to validate my reconstruction against the encoder. Then I need to get the entropy considerations correct so that I actually get some compression out of this format.

• FFmpeg/X264 : Split video mid-GOP without reencoding entire stream

  10 juillet 2012, par yoda_alex

  I've got an H264 video (Stored in an MP4 file). Each GOP is approx 10s long. I want to trim the first couple of seconds off the video, which means I need to split the first GOP. Is there a way to do this without re-encoding the entire video ?

  I've got FFmpeg and x264 available. I'm happy to use either the FFmpeg command line, or my own program linked against ffmpeg of x264 libraries.

  Thanks in advance !