Chapter 23 - Drawing and Writing

Abstract

The most known characteristic creation of Logo is the Turtle; it is a symbolic turtle that moves across the computer screen according to the instruction it receives. With simple instructions like “forward 10” the turtle moves straight ten units, and with “right 90” it turns clockwise to 90 degrees. The statements “pen-up” and “pen-down” specify whether the turtle leaves a track behind it, thus producing drawings or rather just moving to a different location. The language includes many other commands, but these are enough to understand the principles of the Turtle Graphics that made Logo so famous. The idea behind building the turtle is about building a small robotic platform that is able to go forward and backward, turn in place, and lower and raise a pen. Despite this apparent simplicity, if one wants a turtle that works as expected, the task has many stringent requirements that must be adhered to. For instance, the robot must go absolutely straight, the pen must be exactly in the pivoting point of the robot, because it must stay in the same place on the floor while the robot turns, and one needs a tracking system to measure both the traveled distances and angles.