[FFmpeg-user] Disparity in VMAF scores

[Clayton Macleod]

I haven't looked at your files as I don't have the source file you're
using.  I presume this is some public domain test file, is that right?
Anyway, if you use the official vmaf.exe you can output the comparison
results to a file to examine what the score was for each frame.  Scrolling
quickly through those results I suspect you will find that at some point
the frames will no longer be in sync with each other and you will begin to
see a bunch of frames that have a score of zero.  At least that's what I
saw in a project of my own when concatenating several files and using
ffmpeg to do the mkv muxing.  When I started using mkvmerge to do the
muxing my issue went away.  I also had to eliminate all streams except the
video stream to make sure things worked properly.  The presence of audio or
subtitles would sometimes screw things up.  I'm not sure why that is.
Something similar may or may not be happening with your .ts files, but I
don't know.  You can check using ffprobe to see whether or not the total
frame counts match, also.  Naturally, if the total number of frames differs
then something is happening.

On Sun, May 8, 2022 at 3:07 AM Kamaldeep Tumkur <kamaldeep.tumkur at gmail.com>
wrote:

> Hello,
>
> I'm new to the computation of VMAF scores using libvmaf in ffmpeg. While
> trying out generating scores through libvmaf, I have an issue that I cannot
> seem to find an explanation for.
>
> I encoded the 'crowd_run' HQ MP4 with two different encoders. The resulting
> files were compared with the same source (original).
>
> VMAF score of ffmpeg encoded clip:
>
> [libvmaf @ 0x7fc5d8f2cb00] VMAF score: 90.644028
>
>
> Command used:
>
> ffmpeg -i crowd_run_1080p50.mp4 -c:v libx264 -g 100 -keyint_min 100
> -sc_threshold 0 -b:v 12000k -maxrate 15000k -bufsize 15000k -c:a copy -f
> mpegts pass2.ts
>
>
> VMAF score for externally encoded clip:
>
> [libvmaf @ 0x7fc53d729500] VMAF score: 25.168431
>
>
>
> I just don't know how to explain this disparity in generated scores when
> framerates and resolution of the source were retained. The second clip
> doesn't seem to show such a degradation expected through the score.
> Attaching both the encoded clips here for analysis.
>
>
> Could anyone point me to why the external encoder generates an output with
> low VMAF score. Anything in the frame structure?
>
>
> ffmpeg encode:
>
>
> https://drive.google.com/file/d/15-9YyNhYWJoTMxdmESkc7b0mPKKUMqG7/view?usp=sharing
>
>
> external encode:
>
>
> https://drive.google.com/file/d/1Mt2jP51KZ4vTG7SYPnJkCG1uMWqOadid/view?usp=sharing
>
>
>
> Thank you.
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-- 
Clayton Macleod
If no one comes from the future to stop you from doing it, then how bad of
a decision can it really be?