;; -*-scheme-*-
;; Copyright, Alan Horkan 2004.  

(define (script-fu-invert-colours image drawable)
    (gimp-image-undo-group-start image)

    ;; what you see is not always what you get
    ;; if the canvas has been resized the drawable can be offscreen
    ;; making selection gets only onscreen parts of the drawable 

    (if (= (car (gimp-selection-is-empty image)) TRUE)
        (gimp-selection-all image)    
    )

    ;; it is easier to copy, paste and promote than to create a new layer 
    ;; and need to mess about trying to position it.  
  
    ;; copy the original layer or selection    
    (gimp-edit-copy drawable) 
    ;; set this as layer one, even if it is not a proper layer yet ...
    (set! layer-1 (car (gimp-edit-paste drawable FALSE))) 
    (gimp-floating-sel-to-layer layer-1) ;; ... but it is now
    ;; dont want a layer called "Pasted Layer", so change it
    ;(gimp-drawable-set-name layer-1 _"Greyscale Inverse") 

    ;; copy layer
    (set! layer-2 (car (gimp-layer-copy layer-1 TRUE)))
    (gimp-image-add-layer image layer-2 -1)

    ;; good to have layer names, but unnecessary for the final result
    ;(gimp-drawable-set-name layer-2 _"Colour Mode") 

    ;; convert to grayscale/black and white/desaturate 
;    (gimp-desaturate layer-1)
;; turns out this isn't even necessary, the invert is what is important

    ;; invert the colours
    (gimp-invert layer-2)

    ;; add the colours back, colour mode (13) 
    (gimp-layer-set-mode layer-2 13)

    ;; merge the layers temporary layers together into the final layer
    (set! layer-3 (car (gimp-image-merge-down image layer-2 0)))

    ;; give the resulting layer a name
    (gimp-drawable-set-name layer-3 _"Inverted Colours Effect") 

    ;; one could merge again and be left with just the one layer but 
    ;; better to keep the layer for future modifications and adjustment

    (gimp-image-undo-group-end image)
    (gimp-displays-flush)
)

;; alternative location.  put in same place as the other invert
;(set! menu-path _"<Image>/Layer/Colors/") 
;(set! menu-path _"<Image>/Filters/") 
;; incomplete localisation forces me to use American English.  
;(set! menu-path _"<Image>/Filters/Colours/Invert Colours") ; en-US, American English. 
(set! menu-path _"<Image>/Filters/Colors/Invert Colors") ; en, English.  

(set! authors "Alan Horkan.  ")
(set! rights "Alan Horkan, 2004.  ") 
(set! date "2004 04 02 UTC")  

(script-fu-register "script-fu-invert-colours"
    ;; (string-append menu-path) 
    _"<Image>/Filters/Colors/Invert Colors")
    ;; help description
    "Invert Colours but leave the light and dark alone.  " 
    "Alan Horkan"
    rights date "RGB*" 
    SF-IMAGE "Image" 0
    SF-DRAWABLE "Drawable" 0
)

;; see also Tachyon/Invert Light, on which this script was based