Class 19: Turtle Graphics

Held: Wednesday, February 24, 2010

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

Related Pages:

• EBoard.
• Lab: Turtle Graphics.
• Reading: Turtle Graphics.
• Due: Assignment 4: Exploring Colors.

Notes:

• Reading for Friday: Iteration.
• It's Pop Tarts day in CSC 151.
• EC for today's panel on Open Information Culture (4:15 pm Burling).
• EC for Wednesday's gates lecture or Thursday's convo.
• 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).
• Assignment 4 is due. Assignment 5 will be distributed on Friday.

Overview:

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

Modeling the Drawing Process: Turtle Graphics

• We've now seen two (more?) models of images:
  • We can use GIMP-style graphics to select and then do something with the selection.
  • We can use drawings-as-values-style graphics to build composite 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 or 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.