convert mp4 video to gif and optionally remove green background

video-to-gif.sh

#!/bin/bash

#
# convert mp4 video to gif and optionally remove green background
#

# created: early 2025
# ffmpeg version 5.1.6-0+deb12u1
# ImageMagick 6.9.11-60

if [ ! -f "$1" ]; then
  echo "usage: $0 FILE"
  echo "...where FILE is a mp4 FILE"
  exit
fi

MP4FILE="$1"
OUTFILE="output.gif"

# extract frames at this framerate
FPS=10

# delay after a frame in GIF (example: 5 means 20 FPS)
GIF_DELAY=10

REMOVE_GREEN=y

# extract frames (-r 20 means 20 FPS)
ffmpeg -i "$MP4FILE" -vf "scale=1080:-1:flags=lanczos" -r $FPS intermediate_frames_%04d.png

TOTAL=$(ls -1 intermediate_frames_*.png | wc -l)
CNT=1

if [ "$REMOVE_GREEN" = "Y" ]; then

# remove green (adjust the -fuzz parameter if needed)
echo "removing green from pictures..."
for frame in intermediate_frames_*.png; do
  echo "removing green $CNT/$TOTAL"
  CNT=$(expr $CNT + 1)
  convert "$frame" -alpha on -channel RGBA -evaluate Multiply 1 -fill none \
    -fuzz 30% \
    -opaque "#00F000" \
    -opaque "#00F100" \
    -opaque "#00F200" \
    -opaque "#00F300" \
    -opaque "#00F400" \
    -opaque "#00F500" \
    -opaque "#00F600" \
    -opaque "#00F700" \
    -opaque "#00F800" \
    -opaque "#00F900" \
    -opaque "#00FA00" \
    -opaque "#00FB00" \
    -opaque "#00FC00" \
    -opaque "#00FE00" \
    -opaque "#00FF00" \
    "$frame"
done

fi  # if REMOVE_GREEN = y

# put frames to gif
# -delay centiseconds  display the next image after pausing
# convert -delay 10 -loop 0 intermediate_frames_*.png output.gif
echo "merging frames into a gif..."
convert \
  -background none \
  -alpha on \
  -delay $GIF_DELAY \
  -loop 0 \
  -dispose Background \
  intermediate_frames_*.png \
  output.gif
  # -layers OptimizeTransparency \
  # -layers RemoveDups \
  # -layers RemoveZero \

echo "saved in $OUTFILE"
du -h "$OUTFILE"

# remove frames
rm intermediate_frames_*.png