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• Trimming video without encoding

  14 octobre 2020, par Cristian Jorge A. Kusch

  I need to trim a video without encoding into sections that later i can put back together with the concat demuxer of ffmpeg.
I understand that to cut precisely without encoding it is necessary to cut at keyframes, i'm using this command to get the keyframes :

  


  ffprobe -loglevel error -select_streams v:0 -show_entries packet=pts_time,flags -of csv=print_section=0 -input | awk -F',' '/K/ {print $1}'


  


  and they also correspond to the times that Avidemux reports for the keyframes.
I'm using this command to trim the source :

  


  ffmpeg -ss 00:0:2.4607 -noaccurate_seek -avoid_negative_ts 1 -i input.mp4 -to 00:0:3.545 -c copy /content/losslessffmpeg.mp4


  


  But it always ends up with different durations. I would expect that if i cut between keyframe at time 2.460792 and keyframe at time 3.545208 i would get a video of lenght that is the difference between those two times, but it is never the case.
Any help ?
thanks

  


• Crossfade many audio files into one with FFmpeg ?

  3 octobre 2018, par setouk

  Using FFmpeg, I am trying to combine many audio files into one long one, with a crossfade between each of them. To keep the numbers simple, let’s say I have 10 input files, each 5 minutes, and I want a 10 second crossfade between each. (Resulting duration would be 48:30.) Assume all input files have the same codec/bitrate.

  I was pleasantly surprises to find how simple it was to crossfade two files :

  ffmpeg -i 0.mp3 -i 1.mp3 -vn -filter_complex acrossfade=d=10:c1=tri:c2=tri out.mp3

  But the acrossfade filter does not allow 3+ inputs. So my naive solution is to repeatedly run ffmpeg, crossfading the previous intermediate output with the next input file. It’s not ideal. It leads me to two questions :

  1. Does acrossfade losslessly copy the streams ? (Except where they’re actively crossfading, of course.) Or do the entire input streams get reencoded ?

  If the input streams are entirely reencoded, then my naive approach is very bad. In the example above (calling acrossfade 9 times), the first 4:50 of the first file would be reencoded 9 times ! If I’m combining 50 files, the first file gets reencoded 49 times !

  2. To avoid multiple runs and the reencoding issue, can I achieve the many-crossfade behavior in a single ffmpeg call ?

  I imagine I would need some long filtergraph, but I haven’t figured it out yet. Does anyone have an example of crossfading just 3 input files ? From that I could automate the filtergraphs for longer chains.

  Thanks for any tips !

• Is it possible to encode one yuv file to 3 h.264 files with different bitrates with one command ?

  24 juillet 2012, par Richard Knop

  I have a YUV file. I need to encode it to H.264 but using three different bitrates. Is it possible to do it with one command so the yuv file does not need to be processed muttiple times ?

  Here's what I do right now :

  x264 -B 600 -o /path/to/output_first.264 /path/to/input.yuv
  
  x264 -B 800 -o /path/to/output_second.264 /path/to/input.yuv
  
  x264 -B 1000 -o /path/to/output_second.264 /path/to/input.yuv

  Is it possible to do it in one command to make it faster ? YUV file can be quite big so I don't want to extract it three times in a row. And all three encoding processes use the same input YUV file so I guess it should be possible.