Outline 34: Turtle Graphics

Held: Tuesday, 5 April 2016

Summary

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

Related Pages

Overview

Administrivia

Office hours this week
- See http://rebelsky.youcanbook.me.
- Ask me about other available times.
Tutor hours
- Sunday, 3-5 p.m.
- Sunday-Thursday, 7-10 p.m.
Weekly review sessions:
- Wednesday at 8pm in the CS Commons with Evan.
- Thursday 10 am review session with Sam.
- Thursday at 8pm in the CS Commons with Alex.

Reading for Wednesday:
- Iteration
Lab Writeup for Local Recursive Procedure Bindings: Exercise 4b
- Send email titled CSC 151 Lab Writeup 33 (Your Names)
- Do not include the underscores.
- Send to CSC151-02-grader@grinnell.edu
- Due before class on Friday.
Lab Writeup on Turtle Graphics:
- Send email titled CSC 151 Lab Writeup 34 (Your Names)
- Do not include the underscores.
- Send to CSC151-02-grader@grinnell.edu
- Due before class on Wednesday.
Exam 3 to be distributed tomorrow!
Quiz on Friday.

Send your reports to rebelsky@grinnell.edu with subject "CSC 151 Extra Credit". (Do not include the quotation marks.)
Send opportunities to me before class with subject "CSC 151 EXTRA CREDIT OPPORTUNITY!"

Post-break ExCo on British Politics Wednesdays at 8:00 in JRC 203. Starts this week (I think). Just show up; you don't need to sign up.
Meet with your Class Ambassador (aka clambassador) all day Tuesday JRC by dhall. If you can't find him/her/zir/them, try again later.
Fun Japanese Spring Festival Friday at 7pm. Origami and food!

Candlelight Vigil in Solidarity with all those affected by the recent acts of terror around the world. Today at 8 PM in the JRC Lobby.
Pioneer weekend.
Scarlet and Give Back Day Thursday (I will fund your donation of $5 if you find that to be an economic burden)
Host a prospective student.

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.

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.

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.