[FFmpeg-trac] #9204(ffmpeg:new): tpad weird behavior on clips that have been joined with concat

[FFmpeg]

#9204: tpad weird behavior on clips that have been joined with concat
-------------------------------------+-------------------------------------
             Reporter:  fduck        |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  ffmpeg       |                  Version:  git-
             Keywords:               |  master
  filter_complex, filter, tpad,      |               Blocked By:
  concat, filtering                  |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 Summary of the bug:

 The "tpad" filter works fine normally, but when I use it on a clip created
 by joining two or more clips with "concat", it outputs the joined clip
 without any padding.

 Additionally, if I try to use the padded clip by "concat"ing with another
 clip, it gets even weirder: it outputs the original two concat'ed clips,
 with a piece from the end of another clip equal to the number of padding
 frames requested.

 How to reproduce:
 {{{
 ffmpeg version: 4.4
 built on Mac OS Big Sur (11.2.3)

 1. Using tpad on a concat'ed clip:

 % ffmpeg -y -i 'input.mp4' -filter_complex '
 [0:v] split [one][two];
 [one][two] concat [doubled];
 [doubled] tpad=stop=30 [outV]
 ' -map [outV] 'output1.mp4' > output1.txt 2>&1

 Expected output: two copies of input.mp4 with a pad of 30 black frames at
 the end.
 Actual output: two copies of input.mp4 with no pad at the end.


 2. Trying to concat the tpad'ed clip:

 % ffmpeg -y -i 'input.mp4' -filter_complex '
 [0:v] split [one][two];
 [one][two] concat [doubled];
 [doubled] split [one][two];
 [one] tpad=stop=30 [one];
 [one][two] concat [outV]
 ' -map [outV] 'output2.mp4' > output2.txt 2>&1

 Expected output: two copies of input.mp4 followed by 30 black frames
 followed by two more copies of input.mp4.
 Actual output: two copies of input.mp4 followed by the a repeat of the
 final 30 frames of input.mp4.

 }}}

 I tried a number of different input videos from diverse sources, all under
 a minute long, all mp4s in h.264 format.

 I tried inserting ", setpts=PTS-STARTPTS" to reset the presentation
 timestamps after each operation, in case they were causing issues. That
 made no difference in the output.

 Forgive me if this is something caused by user error. I think I understand
 how tpad is supposed to work, but maybe not.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/9204>
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