[FFmpeg-trac] #3809(avcodec:new): Low-resolution MPEG-2 decoding incorrectly

[FFmpeg]

#3809: Low-resolution MPEG-2 decoding incorrectly
-----------------------------------+--------------------------------------
             Reporter:  qyot27     |                     Type:  defect
               Status:  new        |                 Priority:  normal
            Component:  avcodec    |                  Version:  git-master
             Keywords:  mpeg12dec  |               Blocked By:
             Blocking:             |  Reproduced by developer:  0
Analyzed by developer:  0          |
-----------------------------------+--------------------------------------
 '''Summary of the bug:'''
 Low-resolution (here defined as VCD resolution, 352x240) MPEG-2 files are
 being decoded incorrectly in git master.  The effect seems like the
 bottom-most row of macroblocks in the image are smeared.

 It occurs with low-res MPEG-2 streams encoded by HCenc, in which case it
 seems any output plays back incorrectly, regardless of the presence of
 soft pulldown.  Low-res MPEG-2 streams encoded by FFmpeg work on playback
 ''initially'', but if soft pulldown is applied afterward, even these
 streams begin showing the same problem.  On a hardware DVD player the
 playback is correct.

 Using git bisect, the issue was introduced in commit 76b5e99 in January.
 Prior to this commit, the streams decode correctly, and commenting out the
 changes made in that commit from current git master also clears up the
 issue.  Of course, I recognize that the commit was made for a reason, so
 it's probably more that a better check of some kind is necessary.

 '''How to reproduce:'''
 {{{
 $ ./ffmpeg -i ../low_resolution_soft_pulldown.m2v
  ffmpeg version N-65043-ga06c14a Copyright (c) 2000-2014 the FFmpeg
 developers
   built on Jul 28 2014 05:17:31 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)
   configuration: --enable-gpl --enable-version3 --disable-doc
 --optflags=-O0
   libavutil      52. 92.101 / 52. 92.101
   libavcodec     55. 70.100 / 55. 70.100
   libavformat    55. 49.100 / 55. 49.100
   libavdevice    55. 13.102 / 55. 13.102
   libavfilter     4. 11.102 /  4. 11.102
   libswscale      2.  6.100 /  2.  6.100
   libswresample   0. 19.100 /  0. 19.100
   libpostproc    52.  3.100 / 52.  3.100
 [mpeg1video @ 0x9e8db80] Warning MVs not available
 [mpeg1video @ 0x9e8db80] concealing 44 DC, 44 AC, 44 MV errors in I frame
 Input #0, mpegvideo, from '../low_resolution_soft_pulldown.m2v':
   Duration: N/A, bitrate: N/A
     Stream #0:0: Video: mpeg2video (Main), yuv420p(tv), 352x240 [SAR 10:11
 DAR 4:3], max. 2000 kb/s, 29.97 fps, 59.94 tbr, 1200k tbn, 59.94 tbc
 At least one output file must be specified
 }}}

 These particular warnings are repeated extensively during playback in
 ffplay or during conversion:
 [mpeg1video @ 03f5e6e0] Warning MVs not available
 [mpeg1video @ 03f5e6e0] concealing 44 DC, 44 AC, 44 MV errors in I frame

--
Ticket URL: <https://trac.ffmpeg.org/ticket/3809>
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