[FFmpeg-trac] #7353(undetermined:new): Option -ss on output with -c copy uses dts instead of pts

[FFmpeg]

#7353: Option -ss on output with -c copy uses dts instead of pts
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             Reporter:  Misaki       |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:  git-
  undetermined                       |  master
             Keywords:  dts, pts     |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 Editing scenario: have a finished video, want to copy audio from another
 video starting at a certain point.

 If you use the command

 ffmpeg -ss 11 -i video1.webm -i video2.mp4 -c copy output.mkv

 then ffmpeg will seek to the video keyframe before 11 in video1.webm, and
 copy audio from there, instead of from 11.

 If you add -ss 0 to the output, it trims audio before 11. But due to this
 bug, if the first dts of video2.mp4 is negative, it also trims the first
 group of pictures.

 How to replicate:

 ffmpeg -filter_complex color=black -t 2 -g 25  black.mp4; ffmpeg -i
 black.mp4 -c copy -ss 0 black2.mp4; ffmpeg -i black.mp4 -ss 0 black3.mp4

 The first file is 50 frames. The second, using -c copy, is only 25 frames.
 The third, not copying, is 50 frames. Verbose output for these commands
 attached to report.

 The desired behaviour is that packets are trimmed based on their
 presentation timestamp (pts), not their decode timestamp (dts).

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Ticket URL: <https://trac.ffmpeg.org/ticket/7353>
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