Using ffmpeg to split mkv but get a few seconds video lost (cutting between keyframes without re-encoding?)

5

I am using ffmpeg (the real one from FFmpeg, not the fake one from Libav) to split some mkv files:

ffmpeg -i input.mkv -ss 00:01:00 -to 00:02:00 -codec copy output.mkv

However, when I open the output.mkv with VLC, the first few seconds has no video, but only audio. And seconds later I can see both video and audio. The interesting part is, when I try different timestamps after -ss parameter, the duration of "audio only" part varies from 1 second to 4 seconds or even more.

I try to use mkvmerge with --split "timecodes:00:01:00,00:02:00", and the output file works fine, with no "audio only" part at the beginning.

I tried different mkv container with different video/audio encoding in them, but the results are same.

I think I am missing some options with ffmpeg. What should I add to ffmpeg command-line to avoid the several seconds of "audio only" part?

Update 1

Here is the complete output:

$ ffmpeg -i 21.mkv -ss 00:00:58 -to 00:02:00 -codec copy clip.mkv
ffmpeg version 2.2.2 Copyright (c) 2000-2014 the FFmpeg developers
  built on May  7 2014 13:08:45 with gcc 4.8.2 (GCC) 20140206 (prerelease)
  configuration: --prefix=/usr --disable-debug --disable-static --enable-avresample --enable-dxva2 --enable-fontconfig --enable-gnutls --enable-gpl --enable-libass --enable-libbluray --enable-libfreetype --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libv4l2 --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-pic --enable-postproc --enable-runtime-cpudetect --enable-shared --enable-swresample --enable-vdpau --enable-version3 --enable-x11grab
  libavutil      52. 66.100 / 52. 66.100
  libavcodec     55. 52.102 / 55. 52.102
  libavformat    55. 33.100 / 55. 33.100
  libavdevice    55. 10.100 / 55. 10.100
  libavfilter     4.  2.100 /  4.  2.100
  libavresample   1.  2.  0 /  1.  2.  0
  libswscale      2.  5.102 /  2.  5.102
  libswresample   0. 18.100 /  0. 18.100
  libpostproc    52.  3.100 / 52.  3.100
Input #0, matroska,webm, from '21.mkv':
  Metadata:
    encoder         : libebml v1.3.0 + libmatroska v1.4.0
    creation_time   : 2014-05-07 01:47:41
  Duration: 00:42:50.78, start: 0.000000, bitrate: 2833 kb/s
    Stream #0:0(eng): Video: h264 (High), yuv420p(tv, bt709), 1280x720, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
    Stream #0:1: Audio: ac3, 48000 Hz, 5.1(side), fltp, 384 kb/s (default)
Output #0, matroska, to 'clip.mkv':
  Metadata:
    encoder         : Lavf55.33.100
    Stream #0:0(eng): Video: h264 (H264 / 0x34363248), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 23.98 fps, 1k tbn, 1k tbc (default)
    Stream #0:1: Audio: ac3 ([0] [0][0] / 0x2000), 48000 Hz, 5.1(side), 384 kb/s (default)
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
frame= 1414 fps=0.0 q=-1.0 Lsize=   18275kB time=00:01:02.00 bitrate=2414.6kbits/s    
video:15344kB audio:2906kB subtitle:0 data:0 global headers:0kB muxing overhead 0.134916%

Update 2

There may be something to do with "keyframe" stuff, and letting ffmpeg re-encode the video stream will not lead to the few seconds of blank video. However, since mkvmerge can cut the same file between keyframes without re-encoding, I am wondering if there is a way to make ffmpeg do the same work. For example, somewhat "re-define" the keyframe, et cetra.

Zhuoyun Wei

Posted 2014-05-10T03:04:24.800

Reputation: 302

mkvmerge can also split the video at key frames. – Calmarius – 2014-09-18T20:58:12.713

Please include the complete ffmpeg console output. – llogan – 2014-05-10T04:32:38.753

@LordNeckbeard Added. – Zhuoyun Wei – 2014-05-10T04:43:00.343

When you copy the streams, ffmpeg can only copy at keyframes, meaning that you won't always (or, in most cases) get the cut at the exact timestamp you want. Does setting -async 1 as an option work? – slhck – 2014-05-12T05:51:30.660

@slhck Yes it seems there's something to do with keyframes. I tried "-async 1" but the results are the same. Is the fact that ffmpeg can only copy at keyframes means that it cannot cut mkv as precisesly as mkvmerge do? – Zhuoyun Wei – 2014-05-12T10:13:55.870

Answers

0

In order to cut in between keyframes (or GOP's, as they are called), you need to re-encode the video. (the same is true for audio, but there the "frames" are much smaller, usually 1024 samples).

In your case, you could change -codec copy to -acodec copy -vcodec webm to have ffmpeg re-encode your stream. This will obviously take longer, and you will lose quality (if you do it right, this loss will not be noticeable, but there will be some).

Niobos

Posted 2014-05-10T03:04:24.800

Reputation: 146

Thanks. I know re-encoding will solve this problem. But since mkvmerge can split the same video precisely without re-encoding, I am thinking if there is a way to make ffmpeg do the same work. – Zhuoyun Wei – 2014-05-14T02:38:10.820

Asked: 2014-05-10T03:04:24.800

Viewed: 3 150 times

Active: 2020-02-13T09:49:22.313