cycle a layer through all possible hues and save frames (for making a gif!)

rainbow.scm

(define (rainbow img layer dirname)

  (let ( (c 0)
         (dup-layer 0)
         (dup-img 0)
         (to-save 0)
       )

    (set! c -180)

    (while (<= c 180)

      ; duplicate layer
      (set! dup-layer (car (gimp-layer-copy layer 0)))

      ; add duplicate layer to image
      (gimp-image-add-layer img dup-layer 0)

      ; adjust layer
      (gimp-hue-saturation dup-layer 0 c 0 0)   

      ; duplicate image
      (set! dup-img (car (gimp-image-duplicate img)))

      ; merge duplicate image
      (set! to-save (car (gimp-image-merge-visible-layers dup-img 1)))
     
      ; save!
      (let* ( (fname-suffix (number->string (if (< c 0) (+ c 360) c)))
              (fname (string-append dirname "/rainbow" fname-suffix ".png")) 
            )
        (file-png-save-defaults 1 dup-img to-save fname fname)
      )
      ; delete duplicate image
      (gimp-image-delete dup-img)

      ; remove modified layer
      (gimp-image-remove-layer img dup-layer)

      ; increment counter
      (set! c (+ c 1))

    )
  )

)

(script-fu-register 
  "rainbow"
  "Rainbow Magic"
  "Do some rainbow magic."
  "Kendall Stewart"
  "Kendall Stewart"
  "2016"
  "RGB*"
  SF-IMAGE "Image" 0
  SF-DRAWABLE "Layer" 0
  SF-DIRNAME "Directory" "/tmp/"
)

(script-fu-menu-register
  "rainbow"
  "<Image>/Colors"
)

oh yeah, the gif-making code:

# zero-pad the indices to get them all in order
for i in `seq 0 359`; do
  k=$(printf "%03d" ${i})
  mv /tmp/rainbow${i}.png /tmp/rainbow${k}.png
done

# requires imagemagick
convert /tmp/rainbow*.png /tmp/rainbow.gif