image_difference.sf

#!/usr/bin/ruby

#
## https://rosettacode.org/wiki/Percentage_difference_between_images
#

require('Imager')

func img_diff(a, b) {

    func from_file(name) {
         %O<Imager>.new(file => name)
    }

    func size(img) {
        (img.getwidth, img.getheight)
    }

    func pixel_diff(p1, p2) {
        [p1.rgba] »-« [p2.rgba] -> sum_by { .abs }
    }

    func read_pixel(img, x, y) {
        img.getpixel(x => x, y => y)
    }

    func read_line(img, y) {
        img.getscanline(y => y)
    }

    var(img1, img2) = (from_file(a), from_file(b))

    var(w1, h1) = size(img1)
    var(w2, h2) = size(img2)

    if ((w1 != w2) || (h1 != h2)) {
        return nil
    }

    var sum = 0

    for y in (^h1) {

        var *line1 = read_line(img1, y)
        var *line2 = read_line(img2, y)

        [line1, line2].zip {|p1, p2|
            sum += pixel_diff(p1, p2)
        }
    }

    sum / (w1 * h1 * 255 * 3)
}

if (ARGV.len != 2) {
    say "usage: #{File(__MAIN__).basename} [img1] [img2]"
    Sys.exit(1)
}

say 100*img_diff(ARGV[0], ARGV[1])