#!/usr/bin/env bash
set -euo pipefail

if [[ $# -ne 4 ]]; then
  echo "usage: crop-sheet.sh <input> <width> <height> <outdir>"
  exit 1
fi

input="$1"
width="$2"
height="$3"
outdir="$4"

mkdir -p "$outdir"
rm -f "${outdir}"/*.png

magick "$input" -crop "${width}x${height}" +repage "${outdir}/%03d.png"

count=0
for file in "${outdir}"/*.png; do
  [[ -f "$file" ]] || continue
  dims=$(magick "$file" -format "%wx%h" info:)
  if [[ "$dims" != "${width}x${height}" ]]; then
    rm "$file"
    continue
  fi
  mean=$(magick "$file" -format "%[mean]" info:)
  if [[ "$mean" == "0" ]]; then
    rm "$file"
  else
    (( ++count ))
  fi
done

echo "$count"