#!/bin/bash

function helpexit() {
    echo Diff two images.
    echo 'imgdiff "img1" "img2"'
    echo 'If images are of different size, the 2nd image is resized to 1st size'
    exit
}

[[ -z "$1" ]] && helpexit
[[ "$1" = "-h" ]] && helpexit


DISP="display -"
which feh &> /dev/null && DISP="feh --scale-down -"
[[ -z "$DISPLAY" ]] && DISP="echo -n"

size1=$( identify -format "%wx%h" "$1" )
size2=$( identify -format "%wx%h" "$2" )

img2="$2"

if [ ! "$size1" = "$size2" ]; then
  echo "Image sizes different. Resize introduces artefacts. $size2 -> $size1"
  tmp2=$( mktemp --suffix .miff )
  convert "$2" -resize "$size1"\! "$tmp2"
  img2="$tmp2"
  trap "rm -f \"$tmp2\"" 0 1 9 15
fi
diffimg=$( mktemp --suffix .miff )
trap "rm -f \"$tmp2\"" 0 1 9 15
convert -compose difference "$1" "$img2" -composite -auto-level -gamma 2 "$diffimg"

echo -n "RMSE:" >&2
compare -metric RMSE "$1" "$img2" MIFF:- | \
  montage -tile 2x2 -background gray -gravity center \
        -geometry +5+5 "$1" "$img2" \
        - "$diffimg" PNG:- | $DISP
echo ""
rm -f "$diffimg"