Massive artefacts when decoding clearvideo

[Carl Eugen Hoyos]

The low-bitrate samples from ticket #6842 show massive artefacts when decoded with current FFmpeg.

$ ffmpeg -i clv1_100kbps.rm -qscale 2 out.avi
ffmpeg version N-90597-g673604e Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 6.3.0 (GCC)
  configuration: --enable-gpl
  libavutil      56. 13.100 / 56. 13.100
  libavcodec     58. 17.100 / 58. 17.100
  libavformat    58. 10.100 / 58. 10.100
  libavdevice    58.  2.100 / 58.  2.100
  libavfilter     7. 14.100 /  7. 14.100
  libswscale      5.  0.102 /  5.  0.102
  libswresample   3.  0.101 /  3.  0.101
  libpostproc    55.  0.100 / 55.  0.100
Input #0, rm, from 'clv1_100kbps.rm':
  Duration: 00:00:03.20, start: 0.000000, bitrate: 74 kb/s
    Stream #0:0: Video: clearvideo (CLV1 / 0x31564C43), yuv420p, 320x240, 100 kb/s, 15 fps, 15 tbr, 1k tbn, 1k tbc
Please use -q:a or -q:v, -qscale is ambiguous
Stream mapping:
  Stream #0:0 -> #0:0 (clearvideo (native) -> mpeg4 (native))
Press [q] to stop, [?] for help
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf58.10.100
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 320x240, q=2-31, 200 kb/s, 15 fps, 15 tbn, 15 tbc
    Metadata:
      encoder         : Lavc58.17.100 mpeg4
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
frame=   32 fps=0.0 q=2.0 Lsize=     250kB time=00:00:03.20 bitrate= 638.9kbits/s speed= 123x
video:243kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 2.757215%

The output video is recognizable but looks very ugly.

[Carl Eugen Hoyos]

Would you like to elaborate?

[Elon Musk]

Can you prove this is not bug in encoder?

[Carl Eugen Hoyos]

The look of the artefacts indicated an issue with the decoder to me, is there any reason to assume that the files are broken?

[Elon Musk]

Yes. all other files decode ok.

[Carl Eugen Hoyos]

Which other files?

[Carl Eugen Hoyos]

Piotr kindly verified the issue with RealPlayer.