CSC151 2010S, Class 18: Turtle Graphics

Overview:
* Modeling images through process: Turtle graphics.
* Some historical notes.
* Turtle graphics in MediaScheme.
* Lab.

Admin:
* Reading for tomorrow: Iteration.
* EC for Wednesday's panel on Open Information Culture (4:15 pm Burling).
* EC for Thursday's convo (or Wednesday's talk).
* EC for Thursday's CS Extra: Tony Pan on his internship at Microsoft (4:15 3821).
* EC for Friday's CS Table: No Silver Bullet (noon, JRC PDR).
* I know that there are questions on Assignment 4.

Assignment 4

;;; Procedure:
;;;   weights
;;; Parameters:
;;;   steps, a positive integer
;;; Purpose:
;;;   Generate a list of weights between
;;;   0 and 1
;;; Produces:
;;;   waits, a list of real numbers
;;; Preconditions:
;;;   [No additional]
;;; Postconditions:
;;;   The values in waits are "evenly spaced".
;;;   For all reasonable i, element i of waits
;;;     is 1/steps greater than element i-1 of waits.
;;;   waits has (steps+1) elements.
;;;   waits only has values between 0 and 1.
(define weights
  (lambda (steps)
    (map (lambda (n) (/ n steps)) 
         ; "a function that, given n, divide n by steppes"
         (iota (+ steps 1)))))

; Testing: Does the name we use matter?
(define weights1
  (lambda (stupendous)
    (map (lambda (squigglemajig) (/ squigglemajig stupendous)) 
         ; "a function that, given n, divide n by steppes"
         (iota (+ stupendous 1)))))

(define weights2
  (lambda (steps)
    (map / 
         ; "a function that, given n, divide n by steppes"
         (iota (+ steps 1))
         (make-list (+ steps 1) steps))))

(define weights3
  (lambda (steps)
    (map (r-s / steps)
         ; "a function that, given n, divide n by steppes"
         (iota (+ steps 1)))))

But wait!  Where's the (lambda (n) ...) or 
(lambda (x) ...) or (lambda (sqasdfasdf) ...)
in the function we're mapping?
* The (lambda (x) (/ x steps)) is longhand for
  (r-s / steps)
* The computer immediately turns
  (r-s / steps) into (lambda (x) (/ x steps))

Turtle Graphics
* We know three ways of writing algorithms that make pictures
  * GIMP tools: image-select-xxx and then stroke or fill
  * drawings as values
  * Manipulating pixel-by-pixel with image-set-pixel!
* We're adding an Nth: Turtle graphics 
  * Turtle graphics: A simplified drawing system
    Place the pen
    Choose a direction
    Move forward
    Turn
  * A stateful way of thinking about drawing
    * A turtle has a particular
     direction that it is facing.  So, what
     it draws will differ depending on the angle.
    * A turtle has a position
    * A turtle has a color
    * A turtle has a brush

Why do we use this model?
* Simple: Not many operations
* Turns out that using just these operations and
  repetition you can create really cool images
* Historically important


Lab (Hah!)
* Tomorrow