CODEWITHSUNDEEP
ffmpeg
guneykayim
guneykayim

Reputation: 5250

ffmpeg fails to copy correct duration

I want to trim a video with the following command:

ffmpeg.exe -i in.mp4 -ss 76 -t 10 -c copy -an out.mp4

which exactly means, copy 10 seconds after 76 seconds, and don't copy audio. However out.mp4 is 5 seconds length.

This is ffmpeg console output:

ffmpeg.exe -i in.mp4 -ss 76 -t 10 -c copy -an out.mp4
ffmpeg version N-78197-g5893e87 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 5.2.0 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzli
b --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libblu
ray --enable-libbs2b --enable-libcaca --enable-libdcadec --enable-libfreetype --enable-libgme --enab
le-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enab
le-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger
 --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --ena
ble-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --en
able-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --
enable-lzma --enable-decklink --enable-zlib
  libavutil      55. 16.101 / 55. 16.101
  libavcodec     57. 22.102 / 57. 22.102
  libavformat    57. 23.101 / 57. 23.101
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 27.100 /  6. 27.100
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'in.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf54.63.104
  Duration: 00:02:28.54, start: 0.000000, bitrate: 2018 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1056 [SAR 1:1 DAR 20:11]
, 1887 kb/s, 23.98 fps, 23.98 tbr, 90k tbn, 47.95 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 125 kb/s (default
)
    Metadata:
      handler_name    : SoundHandler
Output #0, mp4, to 'out.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.23.101
    Stream #0:0(und): Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 1920x1056 [SAR 1:1 DAR 20:11],
q=2-31, 1887 kb/s, 23.98 fps, 23.98 tbr, 90k tbn, 90k tbc (default)
    Metadata:
      handler_name    : VideoHandler
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
frame=  143 fps=0.0 q=-1.0 Lsize=    1502kB time=00:00:09.96 bitrate=1235.5kbits/s speed= 524x
video:1499kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.194616%

When I analyze this console output I see this inconsistency. It says output time is 9.96 seconds. On the other hand it says frame is 143, so from the latter information I can calculate duration by frame/fps which is 143/23.98=5.96.

Another strange behaviour with this copy command is when I change the start time to something else output video length is varying between 5-10 seconds depending on the start time.

What might be the problem causing these unexpected behaviours?

Upvotes: 3

Views: 1605

Answers (1)

Gyan
Gyan

Reputation: 92928

ffmpeg only cuts at keyframes, so with inter-coded streams, you may not get exact duration. You'll have to transcode the video for a precise cut.

ffmpeg.exe -ss 76 -t 10 -i in.mp4 -an out.mp4

(Placing the -ss and -t before the input will make ffmpeg seek to the position and then decode frames. Placing them afterwards means ffmpeg decodes all frames and then discards till start time is reached)

Upvotes: 3

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