CSC151.01 2006S, Class 35: Manipulating Images with Script-Fu

Admin:
* EC for attending the Dragstravaganza on Friday.
* EC for attending Saturday's football game in drag.
* There are some reports that CSC151 students have been asking 
  TCs and Tutors questions pertaining to the exam.  I will be 
  working to resolve this issue.
* Happy Halloween!
* There were some serious typos in the sample code on the exam.  
  I think they've all been fixed.
* Reading for tomorrow: Drawing with Script-Fu.

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

General Style Questions:
* When should I use a let and not use a let?
  * Pure Scheme programmers say "If it's not intended for use outside
    the main procedure, make it local."
  * For this class, do what makes the most sense for you, unless
    specifically told to use let.

Image Manipulation:
* We can treat an image as a collection of "colors at (x,y)"
* Get the colors with (get-color-at x y)
* Set the color with (set-color-at! x y newcolor)
  * Problem: If the image is visible, you may not see the change.
  * Solution: Change images before showing
* (modify-image! colortrans image)
* Strategy:
  * load image
  * modify it - SLOW!
  * show it