[x265] Artifacts with vbv-init/vbv-end

[Alex Giladi]

Hi Diego,
I would try 0.9 as opposed to 0.5 on both vbv-init and vbv-end.
There is also an optional --vbv-end-fr-adj parameter, you may want to
play with it. May want to tune it, 12+ frames is best at hiding the
stitch point.
Best,
Alex.

On Thu, Oct 29, 2020 at 12:23 PM Diego Pasqualin <dpasqualin at gmail.com> wrote:
>
> Hi, I'm revisiting some parameters in our chunked encoding pipeline and I stumbled upon vbv-init and vbv-end, which seems to be recommended for the use case (https://x265.readthedocs.io/en/3.1/cli.html#cmdoption-vbv-init). However, if I set these two parameters I get really bad artifacts, strangely more intense in the right bottom part of the output video. I took this screenshot as example https://pasteboard.co/JxVhD78.png
>
> This is the full ffmpeg command. If I remove the vbv-init/vbv-end it encodes nicely without artifacts. Any idea on what might be going on? Thank you very much!
>
> ffmpeg -fflags +genpts -i big-buck-bunny.mp4 -map 0:v -bf 3 -refs 3 -crf 21 -force_key_frames expr:gte(t,n_forced*2) -g 50 -keyint_min 50 -x265-params "hrd=1:vbv-init=0.5:vbv-end=0.5:vbv-maxrate=1870:vbv-bufsize=3740:keyint=50:min-keyint=50:no-open-gop=1:bframes=3:ref=3:level=40:limit-modes=1:rect=1:amp=1" -c:v libx265 -flags +cgop -map_chapters -1 -map_metadata -1 -pix_fmt yuv420p -preset medium -profile:v main -r 25 -vsync cfr -sws_flags bicubic -filter:v scale=trunc((oh*iw*sar/ih)/2)*2:720 -an -dn -sn -y ./output.mp4
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