Percentage difference between images

Compute the percentage of difference between 2 JPEG images of the same size. Alternatively, compare two bitmaps as defined in basic bitmap storage.

Useful for comparing two JPEG images saved with a different compression ratios.

You can use these pictures for testing:

The expected difference for these two images is 1.62125%

MAXScript

fn diffImages =
(
	local img1 = selectBitmap caption:"Select Image 1"
	local img2 = selectBitmap caption:"Select Image 2"
	local totalDiff = 0
	for i in 0 to (img1.height-1) do
	(
		local img1Row = getPixels img1 [0, i] img1.width
		local img2Row = getPixels img2 [0, i] img2.width
		
		for j in 1 to img1.width do
		(
			totalDiff += (abs (img1Row[j].r - img2Row[j].r)) / 255.0
			totalDiff += (abs (img1Row[j].g - img2Row[j].g)) / 255.0
			totalDiff += (abs (img1Row[j].b - img2Row[j].b)) / 255.0
		)
	)
	format "Diff: %\%\n" (totalDiff / ((img1.width * img1.height * 3) as float) * 100)
)

OCaml

<ocaml>#! /usr/bin/env ocaml

directory "+glMLite/"
load "jpeg_loader.cma"
load "bigarray.cma"

open Jpeg_loader

let () =

 let img1, width1, height1, col_comp1, color_space1 = load_img (Filename Sys.argv.(1))
 and img2, width2, height2, col_comp2, color_space2 = load_img (Filename Sys.argv.(2)) in

 assert(width1 = width2);
 assert(height1 = height2);
 assert(col_comp1 = col_comp2);  (* number of color components *)
 assert(color_space1 = color_space2);

 let img1 = Bigarray.array3_of_genarray img1
 and img2 = Bigarray.array3_of_genarray img2 in

 let sum = ref 0.0
 and num = ref 0 in

 for x=0 to pred width1 do
   for y=0 to pred height1 do
     for c=0 to pred col_comp1 do
       let v1 = float img1.{x,y,c}
       and v2 = float img2.{x,y,c} in
       let diff = (abs_float (v1 -. v2)) /. 255. in
       sum := diff +. !sum;
       incr num;
     done;
   done;
 done;

 let diff_percent = !sum /. float !num in
 Printf.printf " diff: %f percent\n" diff_percent;

</ocaml>