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• avformat/flvenc : Avoid unnecessary seek

  16 janvier 2020, par Andreas Rheinhardt

  avformat/flvenc : Avoid unnecessary seek
  When shifting the already written data in order to write the keyframe
  
  index, the flv muxer would first store the pre-shift size, then
  
  calculate how big the index will be eventually, then perform some seeks
  
  to update some size fields, then seek back to the end of the file to get
  
  the new position, followed by a seek to the position where writing will
  
  really start. Seeking back to the (already known) end position (that is
  
  actually used to perform this seek) to get the end position is of course
  
  unnecessary. It has been removed.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  
  Signed-off-by : Michael Niedermayer <michael@niedermayer.cc>
  

  • [DH] libavformat/flvenc.c

• FFMPEG : Encoding WEBM with fast-seek and copyts leads to wrong video lenght

  1er février 2020, par TheOverlord2D

  I’m trying to convert a scene from a soft-subbed MKV file into a hard-subbed WEBM file with two-pass. The video encodes fine, but the file shows the wrong length when opened on a media player (it’s showing as if I had encoded the original file from the starting point all the way to the end).

  This is the command I’m using :

  set timestamp=-ss 12:59.069 -to 16:14.277
  
  
  
  ffmpeg -y %timestamp% -copyts -i source.mkv -shortest -c:v libvpx-vp9 -pass 1 -b:v 0 -crf 33 -threads 8 -speed 4 -tile-columns 6 -frame-parallel 1 -an -sn -vf scale=-1:720,subtitles=source.mkv -f webm NUL
  
  ffmpeg -y %timestamp% -copyts -i source.mkv -shortest -c:v libvpx-vp9 -pass 2 -b:v 0 -crf 33 -threads 8 -speed 2 -tile-columns 6 -frame-parallel 1 -auto-alt-ref 1 -lag-in-frames 25 -c:a libopus -b:a 64k -sn -vf scale=-1:720,subtitles=source.mkv -f webm out.webm

  When opening the video in MPC-BE, the video plays regularly until around the point shown on https://i.stack.imgur.com/6bRwc.png (which is where the scene I wanted to cut out ends) then it just skips to the end of the file, and this wrong length is giving me all sorts of issues when I try to use the encoded video.

• Trying to understand the ffmpeg seek cut functionality

  13 février 2020, par user3841429

  I have a video where I would like to cut a part from. The command I use :

  ffmpeg -ss 526.623 -t 347.986 -i 'example.mp4' -c copy -avoid_negative_ts 1 -y res.mp4

  Where expected length is 00:05:47.99. The process log shows the following :

  ...
  
  frame=10582 fps=8258 q=-1.0 Lsize= 1061446kB time=00:05:47.98 bitrate=24987.8kbits/s speed= 272x    
  
  video:1044531kB audio:16556kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.033789%

  As you can see time=00:05:47.98 as expected, but when I play this video in player I see that it has extra 5 seconds of video at the end. I check the playtime length :

  $ ffmpeg -i 'res.mp4' 2>&1 | grep Duration | cut -d ' ' -f 4 | sed s/,//
  
  00:05:53.11

  What the... My clue was the GOP’s I-frame, but when I modify my commands as :

  ffmpeg -ss 526.623 -t 347 -i 'example.mp4' -c copy -avoid_negative_ts 1 -y res.mp4

  Output shows time=00:05:46.99 and actual video length is 00:05:52.13

  In the first case it added 5.13sec, in the second 5.14. Let’s take 5.14 for both cases that show that it has nothing to do with I-frame. There is no magic in this world (I guess). But what then ?

  UPD : It is GOP in the begging of the video. For this video it’s about 7 seconds.