[FFmpeg-trac] #9513(undetermined:new): shortest cuts audio off early on 1fps video

FFmpeg trac at avcodec.org
Tue Nov 16 22:05:15 EET 2021

#9513: shortest cuts audio off early on 1fps video
             Reporter:  Cosmin       |                     Type:  defect
  Stejerean                          |
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
 This might be related to #9512 but the problem manifests itself
 differently and it's specific to low fps video, where it appears the audio
 cuts off as soon as the last frame is written rather than accounting for
 the last frame duration.

 But similarly to #9512 it works differently on video copy vs video

 Let's create the assets
 % ffmpeg -f lavfi -i testsrc2=size=vga:rate=1 -t 10 -y video.mp4
 % ffmpeg -f lavfi -i sine=1000 -t 30 -y music.m4a

 Attempting a simple operation to edit the audio and leave the video alone,
 cutting audio off at the video duration

 % ffmpeg -i music.m4a -i video.mp4 -filter_complex '[0:a:0] volume=1
 [audio]' -map "[audio]" -map 1:v:0 -c:v libx264 -c:a libfdk_aac -shortest
 -y mixed-transcoded.mp4

 This works, produces a 10s output and it plays correctly with audio
 through the end. But the video transcode is unnecessary given the video
 isn't being modified. Trying again with -c:v copy

 % ffmpeg -i music.m4a -i video.mp4 -filter_complex '[0:a:0] volume=1
 [audio]' -map "[audio]" -map 1:v:0 -c:v copy -c:a libfdk_aac -shortest -y

 Based on a naive ffprobe inspection this appears correct with a total
 duration of 10s. But looking at the individual streams show that audio in
 fact is only 7s long (and attempting to play it cuts off audio early).

 I originally assumed this was missing the last 1s due the frame duration,
 but the problem seems potentially worse as it's actually missing almost 3s
 of video
Ticket URL: <https://trac.ffmpeg.org/ticket/9513>
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