[FFmpeg-trac] #8824(avfilter:new): vf_xfade softlocks filtergraph when crossfading different FPS video

[FFmpeg]

#8824: vf_xfade softlocks filtergraph when crossfading different FPS video
-------------------------------------+-------------------------------------
             Reporter:  legraphista  |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  avfilter     |                  Version:  git-
                                     |  master
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 === Summary of the bug: ===

 When cross-fading from a 60fps video to a 5fps video, xfade locks up and
 stops processing frames, but keeps the filtergraph locked

 === Version: ===
 {{{
 ffmpeg version 4.3.1-static https://johnvansickle.com/ffmpeg/  Copyright
 (c) 2000-2020 the FFmpeg developers
   built with gcc 8 (Debian 8.3.0-6)
   libavutil      56. 51.100 / 56. 51.100
   libavcodec     58. 91.100 / 58. 91.100
   libavformat    58. 45.100 / 58. 45.100
   libavdevice    58. 10.100 / 58. 10.100
   libavfilter     7. 85.100 /  7. 85.100
   libswscale      5.  7.100 /  5.  7.100
   libswresample   3.  7.100 /  3.  7.100
   libpostproc    55.  7.100 / 55.  7.100
 }}}

 === How to reproduce: ===
 {{{
 ffmpeg -f lavfi -i testsrc=r=60:d=5 -f lavfi -i testsrc=r=5:d=5
 -filter_complex "
         [1:v] setpts=PTS-STARTPTS,settb=AVTB [v1];
         [0:v] setpts=PTS-STARTPTS,settb=AVTB [v0];
         [v0][v1] xfade=duration=0.5:transition=wipeleft:offset=4.5
 " test.mp4 -y
 }}}

 Another example of such behavior is when cross-fading between a 60fps
 video and a 7fps video, but instead of locking the filter-graph, it ends
 the stream early, outputting a 5.8s video instead of a 9.5s video:
 {{{
 ffmpeg -f lavfi -i testsrc=r=60:d=5 -f lavfi -i testsrc=r=7:d=5
 -filter_complex "
         [1:v] setpts=PTS-STARTPTS,settb=AVTB [v1];
         [0:v] setpts=PTS-STARTPTS,settb=AVTB [v0];
         [v0][v1] xfade=duration=0.5:transition=wipeleft:offset=4.5
 " test.mp4 -y
 }}}

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Ticket URL: <https://trac.ffmpeg.org/ticket/8824>
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