bitrate artifacts with libvpx

[Clément Bœsch]

When encoding a webm/vp8 with default settings, some weird artifacts appear at regular intervals.

☭ ./ffmpeg -loop 1 -i tests/lena.pnm -vf 'scale=2*iw:2*ih' -t 20 -y out.webm
ffmpeg version N-58093-g54af7ce Copyright (c) 2000-2013 the FFmpeg developers
  built on Nov 14 2013 19:11:51 with gcc 4.8.2 (GCC)
  configuration: --enable-gpl --enable-libx264 --enable-libmp3lame --enable-x11grab --enable-libvorbis --samples=/home/ux/fate-samples --enable-libfreetype --enable-libvpx --cpu=native --cc='ccache cc'
  libavutil      52. 53.100 / 52. 53.100
  libavcodec     55. 43.100 / 55. 43.100
  libavformat    55. 21.100 / 55. 21.100
  libavdevice    55.  5.100 / 55.  5.100
  libavfilter     3. 91.100 /  3. 91.100
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
Input #0, image2, from 'tests/lena.pnm':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: ppm, rgb24, 256x256, 25 fps, 25 tbr, 25 tbn, 25 tbc
[libvpx @ 0x181a160] v1.2.0
Output #0, webm, to 'out.webm':
  Metadata:
    encoder         : Lavf55.21.100
    Stream #0:0: Video: vp8 (libvpx), yuv420p, 512x512, q=-1--1, 200 kb/s, 1k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (ppm -> libvpx)
Press [q] to stop, [?] for help
frame=  500 fps=149 q=0.0 Lsize=     434kB time=00:00:20.00 bitrate= 177.9kbits/s    
video:431kB audio:0kB subtitle:0 global headers:0kB muxing overhead 0.898042%

Then play out.webm: at t=5s, t=10s and t=15s, one single frame gets "flashy" (just like if bitrate was wrong/very-low for those frames). It's less visible if bitrate is raised but still visible (try adding -b:v 1M).

It's probably some kind of rate control bug in libvpx, but maybe something is wrong in the wrapper (I don't have time to dig this issue) so I'm opening a ticket.

[Clément Bœsch]

The issue is reproducible with libvpx, I'm closing the issue as it can't be fixed in FFmpeg. See ​http://code.google.com/p/webm/issues/detail?id=717 for details.