Artefacts for damaged h264 stream

[Carl Eugen Hoyos]

A user uploaded a damaged (or incompliant) h264 stream that shows warnings when decoded with the reference decoder but the output looks visually ok, FFmpeg produces artefacts, worse if the decoding error occurs in the first GOP.

$ ffmpeg -i white-car.h264 -qscale 2 out.avi
ffmpeg version N-67839-gc661601 Copyright (c) 2000-2014 the FFmpeg developers
  built on Nov 20 2014 10:04:07 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      54. 14.100 / 54. 14.100
  libavcodec     56. 12.101 / 56. 12.101
  libavformat    56. 14.100 / 56. 14.100
  libavdevice    56.  3.100 / 56.  3.100
  libavfilter     5.  2.103 /  5.  2.103
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, h264, from 'white-car.h264':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: h264 (High), yuv420p, 1280x720, 25 fps, 25 tbr, 1200k tbn, 50 tbc
Please use -q:a or -q:v, -qscale is ambiguous
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf56.14.100
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 1280x720, q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc56.12.101 mpeg4
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> mpeg4 (native))
Press [q] to stop, [?] for help
[h264 @ 0x2db5d20] Cannot use next picture in error concealment
[h264 @ 0x2db5d20] concealing 3047 DC, 3047 AC, 3047 MV errors in P frame
frame=  109 fps=0.0 q=2.0 Lsize=   10351kB time=00:00:04.36 bitrate=19448.9kbits/s
video:10343kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.078848%

[Carl Eugen Hoyos]

Longer sample uploaded to ​http://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket4122/

[Carl Eugen Hoyos]

​http://thread.gmane.org/gmane.comp.video.ffmpeg.user/58729
A user uploaded a sample with artefacts, I uploaded the first seconds to ​http://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket4122/

[Michael Niedermayer]

Fixed in 98a0053d0f90e3309dc1038b1bae3a48bbd9067c

[Carl Eugen Hoyos]

I still see the same artefacts with 98a0053d for white-car_short.h264 as before. They are especially visible when looking at the front door of the first car but several more exist in the top half of the frame. These artefacts are not shown by the reference decoder, the number of output frames differs by one.