automatically overlaying two images, using Gimp

script-fu-overlay.scm

;; script for automatically overlaying two images,
;; for comparison, in gimp.
;;
;; 1. save the file in your scripts directory
;; (/Applications/Gimp.app/Contents/MacOS/Resources/share/gimp/2.0/scripts/ or
;;  /Users/[user]/Library/Application Support/GIMP/2.8/plug-ins on my mac)
;; 2. generate two images to compare
;; 3. run:
;; bash: /Applications/Gimp.app/Contents/MacOS/gimp-2.8 -b '(script-fu-overlay "image1.pdf" "image2.jpg")'
;;
;; image format is flexible, and can be anything gimp will import from 
;; 
;; Copyright 2013, Alex Ausch 
;; Free to use under attribution license: http://creativecommons.org/licenses/by/2.0/ca/

(define (script-fu-overlay img1 img2)
  (let*
      (
       (position1 -1)
       (position2 -2)
       (back-layer nil)
       (front-layer nil)
       (width 0)
       (height 0)
       (image (car (gimp-image-new 10 10 RGB)))
       (drawable (car (gimp-image-get-active-layer image)))
       )
    (set! back-layer(car(gimp-file-load-layer RUN-NONINTERACTIVE image img1)))
    (set! front-layer(car(gimp-file-load-layer RUN-NONINTERACTIVE image img2)))
    (set! width (car (gimp-drawable-width back-layer)))
    (set! height (car (gimp-drawable-height back-layer)))
    (gimp-image-resize image width height 0 0)
    (gimp-image-insert-layer image back-layer 0 position1)
    (gimp-image-insert-layer image front-layer 0 position2)
    (gimp-layer-set-opacity front-layer 40)
    (gimp-display-new image)
    )
  )