Generate "galaxy" gif for Slack emoji.

galaxyizer

#!/bin/bash

set -euo pipefail

trap "rm -f ,$$,*" EXIT

border=3
colors=256
delay=8
frames=24
rainbow=3

while (( $# > 0 ))
do
  case "$1" in
    -b)
      border="$2"
      shift 2
      ;;
    -c)
      colors="$2"
      shift 2
      ;;
    -d)
      delay="$2"
      shift 2
      ;;
    -f)
      frames="$2"
      shift 2
      ;;
    -r)
      rainbow="$2"
      shift 2
      ;;
    *)
      break
      ;;
  esac
done
if (( "$#" != 1 ))
then
  echo <<__END__
Usage: $0 [OPTIONS] FILE

FILE must have alpha channel, and its demension must be 128x128.

  -b N    Increase the width of border added to FILE image.
  -c N    Set the preferred number of colors in the resulting image.
  -d N    Pause (N / 100) seconds before displaying the next frame.
  -f N    Generate N frames for the background animation.
  -r N    Generate N rainbow loops for the background.
__END__
  exit 1
fi


infile="$1"

convert "$infile" -background None -gravity Center -extent 128x128 \
  \( +clone -alpha Extract \
     -morphology Dilate "Disk:$border" -morphology Close Octagon \
     -threshold 50% \
     +duplicate \
     -compose CopyAlpha -composite \) \
  +swap \
  -compose SrcOver -composite \
  ",$$,infile-bordered.png"

convert -size 181x181 radial-gradient: -gravity Center -extent 128x128 +repage \
  -duplicate "$((rainbow - 1))" \
  -compose ModulusAdd -flatten \
  ",$$,bg-base.png"

for i in $(seq 0 "$((frames - 1))")
do
  convert -background Transparent -virtual-pixel Tile \
    \( \
      ",$$,bg-base.png" \
      -channel R -fx "(p+${i}/${frames})%1" \
      -set colorspace HSB \
      -channel GB -evaluate Set 100% \
      -colorspace RGB \
    \) \
    \( \
      ",$$,infile-bordered.png" \
      -colorspace RGB \
    \) \
    -channel RGB -compose SrcOver -flatten \
    -colorspace sRGB \
    ",$$,f-$(printf '%03d' "$i").png"
done

if (( $colors == 256 ))
then
  colors_args=()
else
  colors_args=(-colors $colors)
fi

convert ,$$,f-???.png \
  +repage +dither +map -layers OptimizeTransparency ${colors_args[@]:-} \
  -set delay $delay \
  ${infile/.*/}-galaxy-b$border-c$colors-d$delay-f$frames-r$rainbow.gif