libvpx -crf option is broken

[evilsoup]

If I use a -crf of 1-3, libvpx outputs a broken file. If I use a -crf of 0, or of 4+ the files work, but they are terrible quality with irritating artefacts (I haven't tested the entire range, but 0, 4, 5, 23 and 56 all produce working, albeit horrible files).

Using -crf 0:

evilsoup@enchantment:~/Videos$ ffmpeg -i wonderlove.mov -map 0:v -c:v libvpx -t 30 -crf 0 test0.webm
ffmpeg version git-2012-12-18-b7e085a Copyright (c) 2000-2012 the FFmpeg developers
  built on Dec 20 2012 11:44:19 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --enable-gpl --enable-libfaac --enable-libfdk-aac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-librtmp --enable-libtheora --enable-libvorbis --enable-libvpx --enable-x11grab --enable-libx264 --enable-nonfree --enable-version3 --enable-libass
  libavutil      52. 12.100 / 52. 12.100
  libavcodec     54. 80.100 / 54. 80.100
  libavformat    54. 49.102 / 54. 49.102
  libavdevice    54.  3.102 / 54.  3.102
  libavfilter     3. 28.100 /  3. 28.100
  libswscale      2.  1.103 /  2.  1.103
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
Guessed Channel Layout for  Input Stream #0.1 : stereo
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'wonderlove.mov':
  Metadata:
    major_brand     : qt  
    minor_version   : 537199360
    compatible_brands: qt  
    creation_time   : 2012-03-06 21:55:06
  Duration: 00:04:00.08, start: 0.000000, bitrate: 30340 kb/s
    Stream #0:0(eng): Video: dvvideo (dvcp / 0x70637664), yuv420p, 720x576 [SAR 16:15 DAR 4:3], 28800 kb/s, SAR 59:54 DAR 295:216, 25 fps, 25 tbr, 25 tbn, 25 tbc
    Metadata:
      creation_time   : 2012-03-06 21:55:06
      handler_name    : Apple Alias Data Handler
      timecode        : 01:00:00:00
    Stream #0:1(eng): Audio: pcm_s16le (sowt / 0x74776F73), 48000 Hz, stereo, s16, 1536 kb/s
    Metadata:
      creation_time   : 2012-03-06 21:55:06
      handler_name    : Apple Alias Data Handler
    Stream #0:2(eng): Data: none (tmcd / 0x64636D74)
    Metadata:
      creation_time   : 2012-03-06 21:55:36
      handler_name    : Apple Alias Data Handler
      timecode        : 01:00:00:00
File 'test0.webm' already exists. Overwrite ? [y/N] y
v1.1.0
Output #0, webm, to 'test0.webm':
  Metadata:
    major_brand     : qt  
    minor_version   : 537199360
    compatible_brands: qt  
    encoder         : Lavf54.49.102
    Stream #0:0(eng): Video: vp8, yuv420p, 720x576 [SAR 59:54 DAR 295:216], q=-1--1, 200 kb/s, 1k tbn, 25 tbc
    Metadata:
      creation_time   : 2012-03-06 21:55:06
      handler_name    : Apple Alias Data Handler
      timecode        : 01:00:00:00
Stream mapping:
  Stream #0:0 -> #0:0 (dvvideo -> libvpx)
Press [q] to stop, [?] for help
frame=  750 fps=4.7 q=0.0 Lsize=     965kB time=00:00:30.00 bitrate= 263.4kbits/s    
video:959kB audio:0kB subtitle:0 global headers:0kB muxing overhead 0.631034%

Using -crf 1 (2 and 3 give almost identical output):

evilsoup@enchantment:~/Videos$ ffmpeg -i wonderlove.mov -map 0:v -c:v libvpx -t 30 -crf 1 test1.webm
ffmpeg version git-2012-12-18-b7e085a Copyright (c) 2000-2012 the FFmpeg developers
  built on Dec 20 2012 11:44:19 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --enable-gpl --enable-libfaac --enable-libfdk-aac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-librtmp --enable-libtheora --enable-libvorbis --enable-libvpx --enable-x11grab --enable-libx264 --enable-nonfree --enable-version3 --enable-libass
  libavutil      52. 12.100 / 52. 12.100
  libavcodec     54. 80.100 / 54. 80.100
  libavformat    54. 49.102 / 54. 49.102
  libavdevice    54.  3.102 / 54.  3.102
  libavfilter     3. 28.100 /  3. 28.100
  libswscale      2.  1.103 /  2.  1.103
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
Guessed Channel Layout for  Input Stream #0.1 : stereo
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'wonderlove.mov':
  Metadata:
    major_brand     : qt  
    minor_version   : 537199360
    compatible_brands: qt  
    creation_time   : 2012-03-06 21:55:06
  Duration: 00:04:00.08, start: 0.000000, bitrate: 30340 kb/s
    Stream #0:0(eng): Video: dvvideo (dvcp / 0x70637664), yuv420p, 720x576 [SAR 16:15 DAR 4:3], 28800 kb/s, SAR 59:54 DAR 295:216, 25 fps, 25 tbr, 25 tbn, 25 tbc
    Metadata:
      creation_time   : 2012-03-06 21:55:06
      handler_name    : Apple Alias Data Handler
      timecode        : 01:00:00:00
    Stream #0:1(eng): Audio: pcm_s16le (sowt / 0x74776F73), 48000 Hz, stereo, s16, 1536 kb/s
    Metadata:
      creation_time   : 2012-03-06 21:55:06
      handler_name    : Apple Alias Data Handler
    Stream #0:2(eng): Data: none (tmcd / 0x64636D74)
    Metadata:
      creation_time   : 2012-03-06 21:55:36
      handler_name    : Apple Alias Data Handler
      timecode        : 01:00:00:00
[libvpx @ 0xa03f2e0] v1.1.0
Output #0, webm, to 'test1.webm':
  Metadata:
    major_brand     : qt  
    minor_version   : 537199360
    compatible_brands: qt  
    encoder         : Lavf54.49.102
    Stream #0:0(eng): Video: vp8, yuv420p, 720x576 [SAR 59:54 DAR 295:216], q=-1--1, 200 kb/s, 1k tbn, 25 tbc
    Metadata:
      creation_time   : 2012-03-06 21:55:06
      handler_name    : Apple Alias Data Handler
      timecode        : 01:00:00:00
Stream mapping:
  Stream #0:0 -> #0:0 (dvvideo -> libvpx)
Press [q] to stop, [?] for help
[libvpx @ 0xa03f2e0] Error encoding frame: Invalid parameter
[libvpx @ 0xa03f2e0]   Additional information: cq_level out of range [cfg->rc_min_quantizer..cfg->rc_max_quantizer]
Video encoding failed

I apologise if I've simply misunderstood how to use libvpx, but documentation is pretty sparse.

[slhck]

The error is that the minimum quantizer value is 4 by default. If you set any CRF value below that, it simply doesn't make sense.

Users should be able to override this by setting -qmin to 0, however with the current code this is not possible, since qmin can only be set to anything larger than 0.

libvpxenc.c, line 288 should therefore be:

if (avctx->qmin >= 0)

instead of

if (avctx->qmin > 0)

Because of this bug, the only workaround is to set a -qmin of 1 and, consequently, a CRF of 1 will work.

Also, the check for qmax should include if (avctx->qmax > avctx->qmin) as well, since it can never be lower than qmin.

[cehoyos]

Please consider sending a patch to ffmpeg-devel.

[slhck]

Replying to cehoyos:

Please consider sending a patch to ffmpeg-devel.

I sent a patch that fixes this issue.

Sorry for actually sending two patches -- I had discovered one problem too late and this is my first time contributing.

[cehoyos]

Is this problem fixed in git head?

[slhck]

Yes, this is fixed now.

You can now correctly set qmin to 0 to specify a lower CRF, and if CRF is not within the correct bounds, a more verbose error message will be displayed.

[slhck]

Replying to cehoyos:

Is this problem fixed in git head?

Sorry if you didn't get this ping, anyway this is fixed in Git HEAD.

@evilsoup: If you get bad quality even with low CRF settings, don't forget to choose a higher target (actually, maximum) bit rate. The default is rather low IIRC.