image_diff.sh

#!/bin/bash
# via https://github.com/koyachi/diff_pdf_image/blob/master/diff_pdf_image.rb

# before
image_b=$1
# after
image_a=$2

function to_gray {
    convert $1 -type GrayScale $1.gray.png
}

function to_blue {
    convert $1 +level-colors Blue,White $1.blue.png
}

function to_red {
    convert $1 +level-colors Red,White $1.red.png
}

function layer {
    convert $1 $2 -compose Multiply -composite $image_a.diff.png
}

to_gray $image_b
to_gray $image_a

to_blue $image_b.gray.png
to_red $image_a.gray.png

layer $image_b.gray.png.blue.png $image_a.gray.png.red.png