CSC151 2007S, Class 22: Iterating Over Positions

Admin:
* I've put a new version of DrFu for the Mac on the DrFu web site.  
  This one handles the pixel functions correctly (or so I think).
* Today's lab has a hint on how to more safely create new images.
* Today's lab may also be a bit long.  (Sorry - Lots of fun things to do.)
* Thursday rally at 7:15, Norris, Main, etc.

Overview:
* Iterating over positions.
* Blends and other positionally-computed images.
* Drawing simple shapes.

New idea: Describe images by explicitly giving a formula that computes
a color for each position.
* Drawing a circle:
  *If the point is within radius from the center, draw red
  *Otherwise, draw black

(cond
  ((or (not (defined? 'canvas)) (not (image? canvas)))
   (define canvas (image-new 200 200))
   (image-show canvas)))