[FFmpeg-trac] #9718(undetermined:new): vp8 VBR encoding targets to fixed 256kbit/s rate

[FFmpeg]

#9718: vp8 VBR encoding targets to fixed 256kbit/s rate
-------------------------------------+-------------------------------------
             Reporter:  Bastiaan     |                     Type:  defect
  Welmers                            |
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:  git-
  undetermined                       |  master
             Keywords:  vp8 libvpx-  |               Blocked By:
  vp8                                |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Since version 5.0 the vp8 encoding when only setting -crf and setting -b:v
 0 is suddenly targeted to 256kbit/s. This wasn't the case in previously
 installed version 4.1. Needless to say, 256kbit/s results in very poor
 quality videos.
 I assume this to be a bug, because Variable Bit Rate encodings for libvpx-
 vp8 never needed any targeting bitrate, only a CRF value, the bitrate was
 always advised to set to 0 and it works fine like that. I also cannot find
 a way to unset the 256kbit/s target for VBR mode, removing the `-b:v 0'
 option does not help.

 How to reproduce:
 {{{
 % ffmpeg -i $in -acodec libvorbis -ac 2 -b:a 96k -vcodec libvpx -b:v 0
 -crf 33 -threads 0 -s 854x480 $base.480p-vp8.webm
 ffmpeg version 5.0.1 and git-20220302
 }}}

 In the output, the following message appears, not apearing in 4.1 where
 the bug is not present:
 {{{
 [libvpx @ 0x6b6b780] Bitrate not specified for constrained quality mode,
 using default of 256kbit/sec
 }}}
 Perhaps this bug is introduced here:
 https://github.com/FFmpeg/FFmpeg/commit/14941d386acd90b92aaaf75fbdac2c07d33f167a
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/9718>
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