Held: Monday, 31 March 2014

Back to Outline 32 - Characters and Strings. On to Outline 34 - Iteration.

Summary

We explore another imperative model of images, turtle graphics. In this model, we give drawing expressions to robotic turtles.

Related Pages

• Reading: Turtle Graphics
• Lab: Turtle Graphics
• EBoard 33 (Source 33) (HTML 33) (PDF 33)

Overview

• Modeling images through process: Turtle graphics.
• Some historical notes.
• Turtle graphics in MediaScheme.

Administrivia

• Reading for tomorrow: Iteration
• Homework for Wednesday: HW 7: GitHub
• Today's lab writeup: Extra 2
• Exam 2 returned. Answer keys to be distributed via email.
• New partner assignments: Work with someone in a different class year.
• CS study break tonight (I think)
• CS T shirts soon to be available. KS will talk about eligibilty to vote. (The Curried Lamb-Duh T-shirt is clearly the best)
• Extra credit:
  • Convocation, Wednesday, noon: Elizabeth Colbert (the Sixth Extinction?)
  • CS extras Thursday: ???
  • CS table Friday: TBD

Modeling the Drawing Process: Turtle Graphics

• We've now seen three (more?) models of making images:
  • We can use drawings-as-values-style graphics to build composite images.
  • We can use GIMP-style graphics to select and then do something with the selection.
  • We can manipulate individual pixels
  • We can transform images
• These models permit us to create a variety of interesting drawings.
• However, they do not model how we normally draw, which involves taking pen (or brush) to paper (or canvas).
• The turtle graphics approach to describing images provides a simple model for how we might describe drawings.
• At any point, the person following the instructions has a pen in hand. You need to give the person information on the direction in which to move the pen and the amount to move it. (That's right, no curves here; just lots and lots of straight lines.)
• We separate the two basic operations: You can tell the person drawing to move forward or to turn in a particular direction.
• "It's so simple, even a turtle can do it."
• What if you don't want continuous lines? You can tell the turtle to lift or drop the pen.
• Turtle graphics has been used to control robots that draw.
• Note that turtle graphics, much like GIMP graphics, is an "imperative" model: You give a series of commands to the thing doing the drawing.

Some Historical Notes

Disclaimer: Although I knew much of this information, I did crib some ideas from Wikipedia and the Web for these notes.

• Turtle graphics were invented by Seymour Papert (at MIT) in part of his development of the LOGO programming language. (1960's and beyond)
• LOGO was designed as a computer language intended to help children think better (or at least more algorithmically).
• The original implementation of LOGO did, in fact, have a kind of robot (commonly referred to as a turtle robot) hooked up to a computer. Hence, it made sense for the language to have some basic operations for the robot.
• As computers became more commonplace, it made sense to simulate the turtle on the screen (since not everyone who had a computer would have a turtle robot).
• And it makes sense to show the turtle's path.
• After awhile, drawing on the screen became as interesting as (or more interesting than?) controlling the physical robot.
• The turtle graphics model has persisted, in various forms, over the years.
• Turtle graphics and LOGO are often used in constructionist approaches to teaching. The goal is that students explore freely, starting with a few basic tools and strategies, they come up with their own problems and develop solutions to those problems.

Turtle Graphics in MediaScheme

• Create a new turtle that draws on a particular image with (turtle-new image).
• Move it forward with (turtle-forward! turtle amt)
• Turn it with (turtle-turn! turtle angle)
• Lift the pen with (turtle-up! turtle)
• Put the pen on paper with (turtle-down! turtle)
• Additional operations for people who can't keep track of position and orientation
  • (turtle-teleport! turtle col row)
  • (turtle-face! turtle angle)
  • (turtle-show! turtle)
• You can also set the turtle's brush and color.

Lab

• Make and predict some fun pictures.