Cutting sections with -ss and -t is not frame accurate

Question

I'm using ffmpeg 6.0 to extract small sections from longer videos. According to the doc i can use the -ss option to specify the start time and the -t option to specify the duration and this should result in frame accurate cuts (since FFmpeg 2.1).

However, in my testing i found that the cuts are not frame accurate. I'm using the following command:

ffmpeg -ss 10 -i ffmpeg -ss 10 -i https://storage.googleapis.com/klap-assets/Frame%20Counter%20%5B4K%2C%2060%20FPS%5D%20%E2%80%93%200100.mp4   -t 10 -y -vcodec libx264 -acodec aac -movflags +faststart out2.mp4   -t 10 -y -vcodec libx264 -acodec aac -movflags +faststart out.mp4

This result in a video that start at 9:57 (-2 frames) and ends at 19:57 (-2 frames) instead of 10:00 and 20:00.

Also weird is that ffprobe on the resulting video show its duration being exactly 10 seconds.

FFprobe of the resulting video:

ffprobe version 6.0 Copyright (c) 2007-2023 the FFmpeg developers
  built with Apple clang version 14.0.0 (clang-1400.0.29.202)
  configuration: --prefix=/opt/homebrew/Cellar/ffmpeg/6.0 --enable-shared --enable-pthreads --enable-version3 --cc=clang --host-cflags= --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libaribb24 --enable-libbluray --enable-libdav1d --enable-libmp3lame --enable-libopus --enable-librav1e --enable-librist --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libsvtav1 --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libspeex --enable-libsoxr --enable-libzmq --enable-libzimg --disable-libjack --disable-indev=jack --enable-videotoolbox --enable-neon
  libavutil      58.  2.100 / 58.  2.100
  libavcodec     60.  3.100 / 60.  3.100
  libavformat    60.  3.100 / 60.  3.100
  libavdevice    60.  1.100 / 60.  1.100
  libavfilter     9.  3.100 /  9.  3.100
  libswscale      7.  1.100 /  7.  1.100
  libswresample   4. 10.100 /  4. 10.100
  libpostproc    57.  1.100 / 57.  1.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'out2.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf60.3.100
  Duration: 00:00:10.00, start: 0.000000, bitrate: 303 kb/s
  Stream #0:0[0x1](und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, smpte170m/bt470bg/bt709, progressive), 1920x1080 [SAR 1:1 DAR 16:9], 287 kb/s, 60 fps, 60 tbr, 15360 tbn (default)
    Metadata:
      handler_name    : ISO Media file produced by Google Inc.
      vendor_id       : [0][0][0][0]
      encoder         : Lavc60.3.100 libx264
  Stream #0:1[0x2](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 2 kb/s (default)
    Metadata:
      handler_name    : ISO Media file produced by Google Inc.
      vendor_id       : [0][0][0][0]

I've done some testing with other videos and results are never totally frame accurate.

What am i missing here? How can i get frame accurate cuts with FFmpeg?

Answer 1

Answering 1st question.

It is a well-known issue of ffmpeg itself.

Answering 2nd question.

best result can be achieved when the encoding starts from the keyframe, so start encoding from the timestamp corresponding to the closest keyframe. get timestamp of the nearest keyframe here: https://superuser.com/questions/554620/how-to-get-time-stamp-of-closest-keyframe-before-a-given-timestamp-with-ffmpeg