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• lavfi/delogo : don’t recompute the same difference again and again

  5 juillet 2013, par Jean Delvare

  lavfi/delogo : don’t recompute the same difference again and again
  The top left hand corner pixel coordinates are already stored in
  
  logo_x1 and logo_y1 so don’t recompute each of them 6 times for every
  
  iteration.
  This is a simple code optimization, result is obviously the same. The
  
  performance gain is small (about 2% in my tests) but still good to
  
  have, and the new code is clearer.
  Signed-off-by : Jean Delvare <khali@linux-fr.org>
  
  Reviewed-by ; Stefano Sabatini <stefasab@gmail.com>
  
  Signed-off-by : Michael Niedermayer <michaelni@gmx.at>
  

  • [DH] libavfilter/vf_delogo.c

• Remux and segment only parts of a video file without difference in output

  20 juin 2013, par Christian P.

  I have a working program built on top of libav (alternatively ffmpeg - expertise is either is useful here).

  It takes an mp4 video, encoded with h264 video / AAC audio, and remuxes it to MPEG TS and segments it into X second chunks. It is analogous to the following ffmpeg command :

  ffmpeg -y -i video.mp4 -c:a copy -bsf:a aac_adtstoasc -c:v copy -bsf:v h264_mp4toannexb -flags -global_header -map 0 -f segment -segment_time 10 -segment_list playlist.m3u8 -segment_format mpegts chunk_%03d.ts

  The reason I am not using the command-line, is that I wish to generate only a subset of the segments. So if a video results in 10 segments of between 8 and 12 seconds (the segments are never exactly the desired length due to keyframes), I might wish to generate segments 3-7 at a later time.

  The complete code for my program can be found here.

  I use av_read_frame to read every frame from the source file, remux (including a bitfilter process) and write to output file. Once the duration since the last output becomes close/greater than the desired segment length, I flush the output file, close it, open the next segment and continue.

  I have tried altering the code to do an av_seek_frame to the end of the first segment and start from there (I also attempted to start at the end of the 2nd and 3rd segment). The new segments are the same length (in seconds and pts), but have a different size than the comparable segments from the full runthrough (within a few kilobytes) - the starting segment (whether it's the 2nd, 3rd or other) also shows as having 2 packets LESS than the comparable segment from previously.

  I assumed that av_seek_frame would give me an exact match as if I had manually done a loop with av_read_frame up to that frame, but it seems like it's not the case.

  What I wish for :

  • A way to "fast-forward" in the file to a specific (not approximate) point in the file.
  • To write from that point forward and have the output be completely identical to the output a full run provides (same size, same length, same exact bytes).

• ffmpeg libx264 : whats the difference between crf and profile and preset in terms of quality (bitrate)

  19 août 2016, par Santhosh Yedidi

  In ffmpeg while encoding with libx264 i came across crf, profile and preset

  Whats the difference between them in terms bitrate.

  And if i am using all the three, will they conflict among each other or which one will be effective.