CSC151 2007S, Class 06: Turtle Graphics

Admin:
* Once again, I will miss office hours today.  
  * I should be in my office from 2:15-3:45
* Reading for Friday: Drawings as Values.
* Reminders: 
  * Quiz Friday morning (Scheme and GIMP basics)
  * Next HW to be assigned Friday, due following Wednesday.
* Slight bug introduced into DrFu.  Sorry.  Should go away soon.

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

==Context and More==

* How you describe images
* Simplified instructions for moving a pen
  * Direction
  * Distance
* Can be used for both drawing and for moving to another place to draw.
* "So simple, even a turtle could do it"
* These four operations are traditinoally called "turtle graphics"

* Why we have turtle instructions in the first place: MIT
  * Papert (disciple of Piaget) focusing on "constructivism"
  * Papert built a language (LOGO) that, among other things, let you
    control a robotic turtle
  * Idea: If you don't have the turtle, simulate it on the screen
    * And show where's it been
  * Eventually: Drawing became its own goal

== Key Turtle Operations in Scheme ==

* (turtle-new IMAGE)
* (turtle-forward! TURTLE amt)
* (turtle-turn! TURTLE amt)
* (turtle-up! TURTLE)
* (turtle-down! TURTLE)$a

* (turtle-teleport! TURTLE col row)
* (turtle-face! TURTLE direction)

== Lab ==

===Getting Started===

(define world (image-new 300 200))
(define max (turtle-new world))

===Useful commands===

(turtle-set-brush! TURTLE BRUSH)
(turtle-set-color! TURTLE COLOR)

===Some notes (for next class)===

* Why did you have trouble finding out where turtles start?
* It's dangerous to build new turtles and images in the definitions window.
  * EXAMPLE
* Why did the three triangles get drawn?