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I found a video posted on discord once that made the whole webpage underneath it bounce up and down when it was played. I assume it must have been encoded using a codec that supports a variable resolution, so the y frame height got shorter and longer to make the whole page bounce. I downloaded the video and these are the details:

enter image description here

It appears to be a WebM file. I don't understand how it would have been encoded using a variable resolution though. Any ideas how to do this? Can ffmpeg encode a PNG sequence with variable resolutions for each frame into a single video that also has variable resolution?

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create videos with different resolutions:

ffmpeg -lavfi "color=green:704x396;anullsrc" -t 1 "in-01.webm" -y
ffmpeg -lavfi "color=yellow:640x480;anullsrc" -t 1 "in-02.webm" -y
ffmpeg -lavfi "color=red:800x450;anullsrc" -t 1 "in-03.webm" -y

and concat:

echo "file 'in-01.webm'" > list.txt
echo "file 'in-02.webm'" >> list.txt
echo "file 'in-03.webm'" >> list.txt

ffmpeg -f concat -safe 0 -i "list.txt" -c copy output.webm -y

mpv output.webm
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