CSC151 2007S, Class 39: Manipulating Images with Script-Fu

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Overview:
* Primary image-manipulation procedures.
  * Color transformations.
* Lab.

/In order to modify images, we need .../

* Determine the color at a point
  (get-color-at image x y)
* Choose a new color, perhaps based on that color
  (rgb redpart greenpart bluepart)
  (red color)
  (green color)
  (blue color)
* Set the color at a point in the image
  (set-color-at! image x y newcolor)
* Why is there an exclamation point in set-color-at!  Because it changes
  the underlying image.

* Once we know how to modify one point, we can modify the entire image
  by applying the same procedure at every point in the image
  (modify-image! colorfun image)

/How do we modify colors?/

* Easy way: Anonymously
  (lambda (color) (rgb (do-something-to color) 
                       (do-something-else-to color)
                       (do-something-else-else-to color)))
* But this is verbose, so we might write helpers
  (define redder 
    (lambda (amt color)
      (rgb (+ amt (red color))
           (green color)
	   (blue color))))
* But we can't use this in modify-image!.  We need to fill in only the
  first parameter to get a color transformer.
  * Strategy one: (left-section redder 32)
  * Strategy two: Rewrite redder using a different structure
    (define redder
      (lambda (amt)
        (lambda (color)
          (rgb (+ amt (red color))
               (green color)
	       (blue color)))))a
    (redder 32) => procedure of one parameter that adds 32 to the red
      component
* If you choose good definitions, youc an write interesting transformations
  with fewer words.