something i threw together to make a discord banner out of video frames and will probably want to find and refer to 5 years from now

generate

#!/usr/bin/env bash
set -e
ffmpeg -i mpv-shot%4d.png -filter_complex "\
select=between(n\,20\,39),setpts=PTS-STARTPTS,\
crop=1920:768:0:100,\
scale=600:-1,\
split=2[palette_in][gif];\
[palette_in]palettegen[palette_out];\
[gif]fifo[gif_fifo];\
[gif_fifo][palette_out]paletteuse\
" -y chuu.gif
cp -p chuu.gif chuu.unoptimized.gif
gifsicle -b --no-comments --no-names --no-extensions -d5 -l -O9 chuu.gif

the frame numbers in the between expression are inclusive.
the -d option to gifsicle controls the final framerate of the gif (-d5 => 100/5=20 fps).
the complex filter piping is lifted from https://quinbenson.wordpress.com/2019/07/15/ffmpeg-png-to-animated-gif/