#!/usr/bin/env bash
# Turns a creature illustration on solid black into a transparent aquarium sprite.
#
#   scripts/aquarium-bild.sh bild.png public/aquarium/krake.webp
#
# The illustrations come out of the image generator as RGB on pure black, with no alpha
# channel. "Black becomes transparent" is not enough: the eyes and eyelashes are black
# too, and a creature with see-through eyes looks haunted on a turquoise sea. So the
# background is found as a *region*, not a colour - every dark area big enough to not be
# a facial feature - and only that region is removed.
#
# The edge is the other half. The art was antialiased onto black, so its outline pixels
# are the creature's colour already mixed with black. Cutting them hard leaves a dark
# halo; instead, near the background the opacity follows the brightness and the colour
# is divided back out of the black (un-premultiplied), which is what the mix actually was.
set -euo pipefail

if [[ $# -lt 2 ]]; then
  echo "usage: $0 input.png output.webp [size]" >&2
  exit 1
fi
src=$1
out=$2
size=${3:-512}

tmp=$(mktemp -d)
trap 'rm -rf "$tmp"' EXIT

# How far each pixel is from black: its brightest channel.
magick "$src" -alpha off -channel RGB -separate -evaluate-sequence max "$tmp/hell.png"

# Dark pixels, then drop every dark island under 5000 px back into the body. Measured on
# the first five creatures: the background is one region of ~1M px, the largest facial
# feature is ~2000 px, and nothing sits in between.
magick "$tmp/hell.png" -threshold 8% -negate \
  -define connected-components:area-threshold=5000 \
  -define connected-components:mean-color=true \
  -connected-components 8 "$tmp/grund.png"

# Opacity: solid inside the body, zero out in the background, and in a thin band along
# the outline a ramp on brightness - the band is what catches the antialiased edge. The
# ramp starts a little above black, or the generator's faint noise in the background
# would come back as a faint haze around every creature.
magick "$tmp/grund.png" -morphology Dilate Disk:3 -negate "$tmp/koerper.png"
magick "$tmp/grund.png" -negate -morphology Dilate Disk:5 "$tmp/umriss.png"
magick "$tmp/hell.png" -evaluate Subtract 4% -evaluate Multiply 3.8 "$tmp/rampe.png"
magick "$tmp/rampe.png" "$tmp/koerper.png" -compose Lighten -composite \
  "$tmp/umriss.png" -compose Multiply -composite "$tmp/alpha.png"

# Divide the black back out of the edge colours, attach the opacity, cut to the creature
# and scale down - the sprites are drawn at a few hundred px at most. `-alpha background`
# blanks the colour of fully transparent pixels, which is what lets `-trim` see them as
# border at all.
magick "$src" -alpha off "$tmp/alpha.png" -compose DivideSrc -composite \
  "$tmp/alpha.png" -alpha off -compose CopyOpacity -composite \
  -background none -alpha background -trim +repage -resize "${size}x${size}>" \
  -quality 90 -define webp:alpha-quality=100 "$out"