#!/usr/bin/env bash
# ash-wallpaper.sh — turn a color photo into an Ash-palette wallpaper:
# grayscale body tinted toward the Ash tones, with warm "ember" highlights
# (reds/oranges/golds) selectively preserved. Pleasantville-style.
#
# usage: tools/ash-wallpaper.sh INPUT [OUTPUT]
#
# tuning via environment (defaults shown):
#   PRESET=dark          dark (void->chalk) | light (stone->paper)
#   HUE_MAX=13%          warm band: hue <= HUE_MAX
#   HUE_MIN=94%      ... or hue >= HUE_MIN (catches pure reds)
#   SAT_MIN=12%          below this saturation pixels lose the highlight
#   SAT_MAX=45%          at/above this saturation the highlight is full
#   EMBER_BOOST=130      saturation multiplier (%) applied to highlight areas
#   SENSITIVITY=8x45%    sigmoidal contrast of the final mask
#   BASE_LO= BASE_HI=    override the grayscale tint endpoints
set -euo pipefail

IN=${1:?usage: ash-wallpaper.sh INPUT [OUTPUT]}
OUT=${2:-${IN%.*}-ash.png}
PRESET=${PRESET:-dark}
HUE_MAX=${HUE_MAX:-13%}
HUE_MIN=${HUE_MIN:-94%}
SAT_MIN=${SAT_MIN:-12%}
SAT_MAX=${SAT_MAX:-45%}
EMBER_BOOST=${EMBER_BOOST:-130}
SENSITIVITY=${SENSITIVITY:-8x45%}

if command -v magick >/dev/null; then IM=magick; else IM=convert; fi

case "$PRESET" in
  dark)  BASE_LO=${BASE_LO:-#100f12}; BASE_HI=${BASE_HI:-#f3ede1} ;;
  light) BASE_LO=${BASE_LO:-#b7ae98}; BASE_HI=${BASE_HI:-#f2ecdf} ;;
  *) echo "PRESET must be dark or light" >&2; exit 1 ;;
esac

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

# warmth mask: (hue in warm band) x (saturation ramp)
"$IM" "$IN" -colorspace HSL -channel R -separate "$TMP/hue.png"
"$IM" "$IN" -colorspace HSL -channel G -separate "$TMP/sat.png"
"$IM" "$TMP/hue.png" -threshold "$HUE_MAX" -negate "$TMP/low.png"
"$IM" "$TMP/hue.png" -threshold "$HUE_MIN" "$TMP/high.png"
"$IM" "$TMP/low.png" "$TMP/high.png" -compose Lighten -composite "$TMP/band.png"
"$IM" "$TMP/sat.png" -level "$SAT_MIN","$SAT_MAX" "$TMP/satmask.png"
"$IM" "$TMP/band.png" "$TMP/satmask.png" -compose Multiply -composite \
  -gaussian-blur 0x2 -sigmoidal-contrast "$SENSITIVITY" "$TMP/mask.png"

# ash base + ember-touched highlights, joined by the mask
"$IM" "$IN" \
  \( -clone 0 -modulate 100,0 +level-colors "$BASE_LO","$BASE_HI" \) \
  \( -clone 0 -modulate "100,$EMBER_BOOST" \) \
  -delete 0 \
  \( +clone "$TMP/mask.png" -alpha off -compose CopyOpacity -composite \) \
  -delete 1 -compose Over -composite \
  -colorspace sRGB "$OUT"

echo "wrote $OUT"