normalize size of smartphone screenshots

normalize-screenshot.sh

#!/usr/bin/env bash

set -ue
set -o pipefail

if [[ $# -ne 3 ]]; then
  cat 1>&2 <<END
USAGE: $0 SRC_FILE PHYSICAL_SIZE NEW_FILE

size of DST_FILE (in pixels) will be proportional to PHYSICAL_SIZE (160 DPI)
END
  exit 1
elif ! command -v convert; then
  echo 1>&2 "'convert' program not available'"
  exit 2
fi

input_file="$1"
physical_size="$2"
new_file="$3"

orig_width=$(identify -format "%w" "$input_file")
orig_height=$(identify -format "%h" "$input_file")

orig_dpi=$(echo "sqrt( $orig_width * $orig_width + $orig_height * $orig_height ) / $physical_size " | bc -l)
ratio=$(echo "160.0000 / $orig_dpi" | bc -l)

new_size="$(echo "$ratio * $orig_width" | bc)x$(echo "$ratio * $orig_height" | bc)"

echo "input: ${orig_width}x${orig_height} @ $orig_dpi DPI"
echo "output: $new_size @ 160 DPI"

convert "$input_file" -resize "$new_size" "$new_file"