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• FFMPEG - 2-pass video conversion final file size is larger than expected

  14 juin 2022, par Kevin Korb

  We are converting files and the target filesize is important. For one version of the video it can not be above 750KB.

  


  We are using the 2-pass encoding and specifying the target audio bitrate and target video bitrate, however the longer the duration of the video, the larger the file becomes and exceeds our 750KB target.

  


  


  For example, our file is 53.3 seconds, we know our final target of
750KB. We like our audio at 48kbps (mono).

  


  So we calculate our target (total) bitrate at : 750*8/53.3 = 112.57
total kbps Audio bitrate = 48kbps Video bitrate = 112-48 = 64kbps
video

  


  We would expect the file produced to be 746.2KB. (112*53.3 / 8)

  


  The file produced is 850k(ish).

  


  


  Is there something that takes up space in the file that isn't video or audio that we need to take into consideration ?

  


• Why is the resulting clip of my ffmpeg concatenation much longer than expected ?

  21 mai 2018, par joeycato

  I’d like to extract a set of clips from a 60fps input video, change each of their durations, then recombine those modified clips. Unfortunately when I attempt this, the resulting clip doesn’t exactly match the sum of those durations.

  Here’s my repro case :

  1. Extend an extracted 2-second clip to 4 seconds ( input frame count = 120 = 60fps * 2 sec ) )

  ffmpeg -y -ss 8 -t 2 -i ./clip_60fps.avi -filter:v "settb=(4/120),setpts=(4/120)*N/TB" -r 120/4 -vcodec huffyuv step1.avi

  2. Shorten an extracted 4-second clip to 1 second ( input frame count = 240 = 60fps * 4 sec )

  ffmpeg -y -ss 10 -t 4 -i ./clip_60fps.avi -filter:v "settb=(1/240),setpts=(1/240)*N/TB" -r 240/1 -vcodec huffyuv step2.avi

  3. Concatenate the modified clips :

  mylist.txt contents :

  file './step1.avi'
  
  file './step2.avi'

  ffmpeg -y -safe 0 -f concat -i ./mylist.txt -r 60 -vcodec huffyuv steps_joined.avi

  The result :

  ffmpeg -i step1.avi 2>&1 | grep Duration
  
  ffmpeg -i step2.avi 2>&1 | grep Duration
  
  ffmpeg -i steps_joined.avi 2>&1 | grep Duration

  Duration : 00:00:04.00, start : 0.000000, bitrate : 3228 kb/s

  Duration : 00:00:01.00, start : 0.000000, bitrate : 23132 kb/s

  Duration : 00:00 :39.98, start : 0.000000, bitrate : 910 kb/s

  Why am I not getting an exact result of 5 seconds, but instead a much larger value ?

• Constant Frame Rate Video extracting more frames than expected - FFMPEG

  22 septembre 2023, par superand12

  I'm trying to extract 1 frame per second in a 3 second video that has a video frame rate of 30 fps.

  


  ffmpeg -i 3seconds.mp4 -r 1 -q:v 2 output_%04d.jpg

  


  I noticed that this command was returning 5 frames instead of 3. I then run this command to figure out that there are iframes in the video (not sure if thats the reason).

  


  ffprobe -v error -select_streams v:0 -show_frames -show_entries frame=pict_type -of csv 3seconds.mp4

  


  This may have been redundant but adding this filter flag gave me exactly 3 frames. Unclear about the underlying frame selection process.

  


  ffmpeg -i 3seconds.mp4 -filter:v fps=1 -r 1 -q:v 2 output_%04d.jpg

  


  Video info :

  


  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '3seconds.mp4': Metadata: major_brand     : isom minor_version   : 512 compatible_brands: isomiso2avc1mp41 encoder         : Lavf58.20.100 Duration: 00:00:03.07, start: 0.000000, bitrate: 2753 kb/s Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc), 1920x1080 [SAR 1:1 DAR 16:9], 2747 kb/s, 30 fps, 30 tbr, 15360 tbn, 60 tbc (default) Metadata: handler_name    : GoPro AVC vendor_id       : [0][0][0][0] timecode        : 19:56:41:04 Stream #0:1(eng): Data: none (tmcd / 0x64636D74), 0 kb/s Metadata: handler_name    : GoPro AVC timecode        : 19:56:41:04