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• How to extract time-accurate video segments with ffmpeg ?

  25 mai 2016, par Jim Miller

  This is not a particularly new question area around here, but I’ve tried what’s been suggested there without much luck. So, my story :

  I’ve got a hunk of 15 seconds of straight-from-the-camera.mov video out of which I want to extract a specific chunk, which I can identify by start time and stop time, in seconds. I started by trying to do what I’ll call a "copy extraction" : to get seconds 9 to 12,

  ffmpeg -i test.mov -vcodec copy -acodec copy -ss 9 -to 12 test-copy.mov

  This was a not-bad start, but there are some black frames at the beginning and end of the clip, which I can’t have — it has to be a clean edit from the original. So, I tried recoding the original into a new, trimmed clip :

  ffmpeg -i test.mov -ss 00:00:09 -t 00:00:03 test-out.mov

  This is better, but not quite : There are no longer any black frames at the beginning of the clip, but they’re still there at the end.

  After some more browsing and reading, I then suspected that the problem is that ffmpeg is having trouble finding the proper points because of a lack of keyframes in the original video. So I recoded the original video to (presumably) add keyframes, in a couple of different ways. Since I want to be able to pick video at boundaries of a second ("from 9 seconds to 12 seconds"), I tried, copying various suggestions around the web,

  ffmpeg -i test.mov -force_key_frames "expr:gte(t, n_forced)" test-forced.mp4

  and

  ffmpeg -i test.mov -g 1 test-g-inserted.mp4

  (I built these as mp4’s based on some comments about an mp4 container being needed to support the keyframe search, but I’m honestly just hacking here.) I then tried the extraction as before, but on these new videos that presumably now have keyframes in them. No luck — both seem to be about the same ; the start is OK but there are still black frames at the end. (FWIW, both test-forced.mp4 and test-g-inserted.mp4 also have trailing black frames.)

  So : I’m still stuck, and would like to not be. Any insights out there as to what I’m doing wrong ? I feel like I’m close, but I really need to get rid of those trailing black frames....

• Revision 6491065775 : Remove the redundant index computation in the first pass Remove the redundant i

  29 juillet 2014, par Pengchong Jin

  Changed Paths :
   Modify /vp9/encoder/vp9_firstpass.c

  
  
  Remove the redundant index computation in the first pass

  Remove the redundant index computation when store the first
  pass block-wise statistics. Currently, a single byte is
  allocated for a 16x16 blocks, and all the frame statistics
  saved during the first pass will be kept in memory for use
  in the second pass. For a 1920x1080 300-frame clip, it will
  take about 2.3 MB memory. This feature is off in current
  setting.

  Change-Id : I135a95b348ec093d54c6a07e1e8237626909e3bd

• Revision 9d9b70a36a : Allow backward prob update in external mode info coding flow This commit enable

  21 juillet 2014, par Jingning Han

  Changed Paths :
   Modify /vp9/encoder/vp9_encodeframe.c

  
  
  Allow backward prob update in external mode info coding flow

  This commit enables vpxenc to properly count the coded motion
  vector related information for backward update. This allows the
  coding flow using external mode info to use backward probability
  update. In the short test clip, over 10% bit-rate saving is
  observed at no distortion change.

  Change-Id : Ie27e97114ab91c3d95ba7b5554d617d226db5e20