Turtle Graphics

Turtle graphics is a vector graphics system using a relative cursor (the turtle) on a cartesian plane.

The turtle has three properties: a position, a heading (direction), and a pen.

The turtle can be moved using commands that are relative to its current state (position and heading).

The turtle carries a pen that can ben set down on the paper, lifted up, and change color.

Each time the turtle moves and the pen is down on the paper it will draw a line on its path.

Turtle graphics was popularized by the Logo programming language. This Xcode playground provides a Swift implementation of a similar system. We use an octopus to represent the turtle because it is cuter and also because it can actually spray ink.

Turtle movement commands:

• forward(distance): move forward in the current direction by the indicated distance
• back(distance): move backwards in the current direction by the indicated distance
• left(degrees): turn left by the indicated angle
• right(degrees): turn right by the indicated angle
• home(): move the turtle to the starting position and initial heading (at center of the paper, pointing up)

Pen commands:

• penUp(): lift the pen up from the paper
• penDown(): set the pen down on the paper
• setColor(color): change the color of the pen. Colors can be created using the function TurtleColor.from(hex:), a few predefined colors are provided by the system: TurtleColor.black, TurtleColor.white, TurtleColor.blue, TurtleColor.green, TurtleColor.red, TurtleColor.yellow

The turtle can be show or hidden without affecting its drawing (if the turtle is hidden and the pen is down it will still draw):

• hide()
• show()