原始 png frames / 24fps / 預設 profile 壓縮 => 得 1.2 MB 
ffmpeg -framerate 24 \
  -i frames/gacha_%04d.png \
  -c:v libvpx-vp9 \
  -pix_fmt yuva420p \
  default.webm
bg-white bg-black
原始 png frames / 24fps / b:v 1.5M 壓縮 => 得 932 KB 
ffmpeg -framerate 24 \
  -i frames/gacha_%04d.png \
  -c:v libvpx-vp9 \
  -b:v 1.5M \
  -pix_fmt yuva420p \
  b1500.webm
bg-white bg-black
png 去 alpha 轉黑底 jpg / 24fps / 預設 profile 壓縮 => 得 713 KB 
cp -r frames jpg_frames;
magick mogrify -format jpg \
  -background black \
  -alpha remove \
  jpg_frames/gacha_*.png;
rm -f jpg_frames/gacha_*.png;

ffmpeg -framerate 24 \
  -i jpg_frames/gacha_%04d.jpg \
  -c:v libvpx-vp9 \
  -pix_fmt yuv420p \
  default_jpg.webm
bg-white bg-black
png 去 alpha 轉黑底 jpg / 24fps / b:v 800k 壓縮 => 得 253 KB 
cp -r frames jpg_frames;
magick mogrify -format jpg \
  -background black \
  -alpha remove \
  jpg_frames/gacha_*.png;
rm -f jpg_frames/gacha_*.png;

ffmpeg -framerate 24 \
  -i jpg_frames/gacha_%04d.jpg \
  -c:v libvpx-vp9 \
  -b:v 800k \
  -pix_fmt yuv420p \
  b800_jpg.webm
bg-white bg-black