[FFmpeg-user] blotchy encoding on stationary areas

[Jason C]

At first glance I wonder if you are "over-optioning", perhaps try trimming
down then observing the effects of various options. If you're going to be
experimenting with parameters, you have a lot of variables in your current
parameter set, it'll be easy to get confused.

Personally I don't know much about vp9 but the gop size (360) feels high,
but take that with a grain of salt.

In my own minimal tests just now I achieved good 2-pass results with videos
with stationary areas using just (note that the default gop size for vp9 is
actually much higher than 360, the below outputs ended up with just a
single I frame at the start -- in my tests varying it did not affect the
output quality) (also, my PC is too slow to do "-speed 0" in a reasonable
amount of time):

  ffmpeg -i %1 -pass 1 -b:v 2M -vcodec libvpx-vp9 -speed 4 -f webm -y %2
  ffmpeg -i %1 -pass 2 -b:v 2M -vcodec libvpx-vp9 -speed 1 -f webm -y %2

But at the same time, 2M was more than enough for my videos (they were
coming out around 1.8 anyways). Can you post a sample video, or at least
tell what your input video dimensions and frame rates are?

Please do not take my words here as any sort of authority. There are others
on this list with far more knowledge, I just happened to reply first.

Jason


On Mon, Feb 1, 2016 at 11:31 AM, Matt Zagrabelny <mzagrabe at d.umn.edu> wrote:

> Greetings ffmpeg folks,
>
> I have been using ffmpeg to encode DVDs to webm files (vp9). Things
> look really good in general, however it seems that when the scene is
> not changing much, such as a person talking and the camera doesn't
> move off of them, then the walls in the scene will get encoded with
> very big blotchy squares.
>
> Here are my encoding commands:
>
> ffmpeg -i INPUT.vob -vf yadif -pass 1 -passlogfile first-pass-log-file
> -b:v 2M -auto-alt-ref 1 -quality best -minrate 100K -maxrate 16M
> -lag-in-frames 16 -keyint_min 0 -g 360 -skip_threshold 0 -qmin 0 -qmax
> 60 -vcodec libvpx-vp9 -f webm -- - &> /dev/null
>
> ffmpeg -i INPUT.vob -vf yadif -pass 2 -passlogfile first-pass-log-file
> -b:v 2M -auto-alt-ref 1 -quality best -minrate 100K -maxrate 16M
> -lag-in-frames 16 -keyint_min 0 -g 360 -skip_threshold 0 -qmin 0 -qmax
> 60 -vcodec libvpx-vp9 -acodec libvorbis -aq 3 OUTPUT.webm
>
> Any suggestions on things to change to get rid of those blotchy areas?
>
> I was thinking of maybe upping the minrate. I know I can try
> experimenting with the encoding parameters, but I was hoping for a
> little direction in what to perhaps perturb.
>
> Thanks for any help!
>
> -m
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